Surah Sheba, Gratitude emphasis and why reward accusation is replied here in unique way.
The first verse starts with praising God, and emphasizing praise belongs to him in this world as well as the next which is unique also:
الْحَمْدُ لِلَّهِ الَّذِي لَهُ مَا فِي السَّمَاوَاتِ وَمَا فِي الْأَرْضِ وَلَهُ الْحَمْدُ فِي الْآخِرَةِ ۚ وَهُوَ الْحَكِيمُ الْخَبِيرُ | All praise belongs to Allah to whom belongs whatever is in the heavens and whatever is in the earth. To Him belongs all praise in the Hereafter, and He is the All-wise, the All-aware. | Saba : 1
The next verses emphasize on nature of deeds, disbelief and belief, and this is found throughout Quran, so will skipped, as they are not proof for this theme I'm talking about.
وَيَرَى الَّذِينَ أُوتُوا الْعِلْمَ الَّذِي أُنْزِلَ إِلَيْكَ مِنْ رَبِّكَ هُوَ الْحَقَّ وَيَهْدِي إِلَىٰ صِرَاطِ الْعَزِيزِ الْحَمِيدِ | Those who have been given knowledge see that what has been sent down to you from your Lord is the truth and [that] it guides to the path of the All-mighty, the All-laudable. | Saba : 6
Notice, "Praised" (hameed) is the end, and so praising God is of the path that is straight.
Disbelief and belief is mentioned, but this throughout Quran, what is important is how it's linked here to gratitude and ingratitude....
So skipping, we go to God's favor upon Dawood (a) and his grace on him is a favor to mankind:
وَلَقَدْ آتَيْنَا دَاوُودَ مِنَّا فَضْلًا ۖ يَا جِبَالُ أَوِّبِي مَعَهُ وَالطَّيْرَ ۖ وَأَلَنَّا لَهُ الْحَدِيدَ
Surat Saba : 10 سورة سبإ : ١٠
Certainly We gave David our grace: ‘O mountains and birds, chime in with him!’ And We made iron soft for him,
Then we see reminder of Solumain (a) and favor on him:
يَعْمَلُونَ لَهُ مَا يَشَاءُ مِنْ مَحَارِيبَ وَتَمَاثِيلَ وَجِفَانٍ كَالْجَوَابِ وَقُدُورٍ رَاسِيَاتٍ ۚ اعْمَلُوا آلَ دَاوُودَ شُكْرًا ۚ وَقَلِيلٌ مِنْ عِبَادِيَ الشَّكُورُ
Surat Saba : 13 سورة سبإ : ١٣
They built for him as many temples as he wished, and figures, basins like cisterns, and caldrons fixed [in the ground]. ‘O House of David, act thankfully, and few of My servants are grateful.’
So we can see emphasis is on gratitude which few servants up to that time or during that time were.
Then we see an example of a city expected to show gratitude for favors of another kind:
لَقَدْ كَانَ لِسَبَإٍ فِي مَسْكَنِهِمْ آيَةٌ ۖ جَنَّتَانِ عَنْ يَمِينٍ وَشِمَالٍ ۖ كُلُوا مِنْ رِزْقِ رَبِّكُمْ وَاشْكُرُوا لَهُ ۚ بَلْدَةٌ طَيِّبَةٌ وَرَبٌّ غَفُورٌ
Surat Saba : 15 سورة سبإ : ١٥
There was certainly a sign for Sheba in their habitation: two gardens, to the right and to the left. ‘Eat of the provision of your Lord and give Him thanks: a good land and an all-forgiving Lord!’
فَأَعْرَضُوا فَأَرْسَلْنَا عَلَيْهِمْ سَيْلَ الْعَرِمِ وَبَدَّلْنَاهُمْ بِجَنَّتَيْهِمْ جَنَّتَيْنِ ذَوَاتَيْ أُكُلٍ خَمْطٍ وَأَثْلٍ وَشَيْءٍ مِنْ سِدْرٍ قَلِيلٍ | But they disregarded [the path of Allah], so We unleashed upon them a violent flood and replaced their two gardens with two gardens bearing bitter fruit, tamarisk, and sparse lote trees. | Saba : 16
Then comes this verse which shows how much emphasis there is on this theme:
ذَٰلِكَ جَزَيْنَاهُمْ بِمَا كَفَرُوا ۖ وَهَلْ نُجَازِي إِلَّا الْكَفُورَ | We requited them with that for their ingratitude. Do We not requite ingrates? | Saba : 17
And we see patience and gratitude side by side:
فَقَالُوا رَبَّنَا بَاعِدْ بَيْنَ أَسْفَارِنَا وَظَلَمُوا أَنْفُسَهُمْ فَجَعَلْنَاهُمْ أَحَادِيثَ وَمَزَّقْنَاهُمْ كُلَّ مُمَزَّقٍ ۚ إِنَّ فِي ذَٰلِكَ لَآيَاتٍ لِكُلِّ صَبَّارٍ شَكُورٍ | But they said, ‘Our Lord! Make the stages between our journeys far apart,’ and they wronged themselves. So We turned them into folktales and caused them to disintegrate totally. There are indeed signs in that for every patient and grateful [servant]. | Saba : 19
We see Shirk here is condemned from gratitude to Creator perspective:
قُلِ ادْعُوا الَّذِينَ زَعَمْتُمْ مِنْ دُونِ اللَّهِ ۖ لَا يَمْلِكُونَ مِثْقَالَ ذَرَّةٍ فِي السَّمَاوَاتِ وَلَا فِي الْأَرْضِ وَمَا لَهُمْ فِيهِمَا مِنْ شِرْكٍ وَمَا لَهُ مِنْهُمْ مِنْ ظَهِيرٍ | Say, ‘Invoke those whom you claim [to be gods] besides Allah! They do not control [even] an atom’s weight in the heavens or the earth, nor do they have any share in [either of] them, nor is any of them His helper.’ | Saba : 22
And as a provider:
قُلْ مَنْ يَرْزُقُكُمْ مِنَ السَّمَاوَاتِ وَالْأَرْضِ ۖ قُلِ اللَّهُ ۖ وَإِنَّا أَوْ إِيَّاكُمْ لَعَلَىٰ هُدًى أَوْ فِي ضَلَالٍ مُبِينٍ | Say, ‘Who provides for you from the heavens and the earth?’ Say, ‘Allah! Indeed either we or you are rightly guided or in manifest error.’ | Saba : 24
Mohammad (s) as a universal favor to mankind:
وَمَا أَرْسَلْنَاكَ إِلَّا كَافَّةً لِلنَّاسِ بَشِيرًا وَنَذِيرًا وَلَٰكِنَّ أَكْثَرَ النَّاسِ لَا يَعْلَمُونَ | We did not send you except as a bearer of good news and warner to all mankind, but most people do not know. | Saba : 28
On this theme, it talks about the influence of the powerful on the weak and shows Mohammad (s) is a means out of that cycle of misguidance, and then emphasizes:
وَمَا أَرْسَلْنَا فِي قَرْيَةٍ مِنْ نَذِيرٍ إِلَّا قَالَ مُتْرَفُوهَا إِنَّا بِمَا أُرْسِلْتُمْ بِهِ كَافِرُونَ | We did not send a warner to any town without its affluent ones saying, ‘We indeed disbelieve in what you have been sent with.’ | Saba : 34
And then emphasizes on wealth deceiving them:
وَقَالُوا نَحْنُ أَكْثَرُ أَمْوَالًا وَأَوْلَادًا وَمَا نَحْنُ بِمُعَذَّبِينَ | They say, ‘We have greater wealth and more children, and we will not be punished!’ | Saba : 35
Then emphasizes on true sustenance from God:
قُلْ إِنَّ رَبِّي يَبْسُطُ الرِّزْقَ لِمَنْ يَشَاءُ وَيَقْدِرُ وَلَٰكِنَّ أَكْثَرَ النَّاسِ لَا يَعْلَمُونَ | Say, ‘Indeed my Lord expands the provision for whomever He wishes and He tightens it, but most people do not know.’ | Saba : 36
Then compares the two, to show why lower and less rich should still be grateful:
وَمَا أَمْوَالُكُمْ وَلَا أَوْلَادُكُمْ بِالَّتِي تُقَرِّبُكُمْ عِنْدَنَا زُلْفَىٰ إِلَّا مَنْ آمَنَ وَعَمِلَ صَالِحًا فَأُولَٰئِكَ لَهُمْ جَزَاءُ الضِّعْفِ بِمَا عَمِلُوا وَهُمْ فِي الْغُرُفَاتِ آمِنُونَ | It is not your wealth, nor your children, that will bring you close to Us in nearness, excepting those who have faith and act righteously. It is they for whom there will be a twofold reward for what they did, and they will be secure in lofty abodes. | Saba : 37
Then emphasizes on wealth being used as a means to true sustenance and God's proximity again:
قُلْ إِنَّ رَبِّي يَبْسُطُ الرِّزْقَ لِمَنْ يَشَاءُ مِنْ عِبَادِهِ وَيَقْدِرُ لَهُ ۚ وَمَا أَنْفَقْتُمْ مِنْ شَيْءٍ فَهُوَ يُخْلِفُهُ ۖ وَهُوَ خَيْرُ الرَّازِقِينَ | Say, ‘Indeed my Lord expands the provision for whomever of His servants that He wishes and tightens it, and He will repay whatever you may spend, and He is the best of providers.’ | Saba : 39
We see emphasis on the favor of Mohammad (S) but then finally we see the reward verse is best suited here:
قُلْ مَا سَأَلْتُكُمْ مِنْ أَجْرٍ فَهُوَ لَكُمْ ۖ إِنْ أَجْرِيَ إِلَّا عَلَى اللَّهِ ۖ وَهُوَ عَلَىٰ كُلِّ شَيْءٍ شَهِيدٌ | Say, ‘Whatever reward I have asked you so it is for your own good. Surely my reward is only upon Allah, and He is witness to all things.’ | Saba : 47
Given how much the Surah emphasized on gratitude, we see the reward verse emphasizing it's for our sake, has a best place here because of the continuous theme in that regard!