Salam,
The Jews believe God is One. They believe in Torah. They also believe in the leaders who came after Moses.
So, according to Quran, why believing in all these, is still not enough? What was the need for another Messenger (Muhammad), and another Book?
Salam
To end Nubuwa is not an easy task, but the world is coming to an end. All good things must come to an end, and this trial of humanity is coming to a finality.
You ask a good question. Musa (a) prays in the Torah "for the one who you will send", God get's angry at him and says "You have your brother Aaron". What does this mean?
It means two things. One that God doesn't want Musa (a) thinking about the future, but the now. And make his people enlightened and relieve his knot from his tongue through Aaron (a) as opposed to the one you will send.
But did God relieve the knot on Moses' (a) tongue at the end? No, because people didn't honor Aaron (a) and so they didn't become enlightened.
To this day, the Torah both praises Aaron (a) and his family (a), but also condemns Aaron (a) and disconnects the chosen ones from his family. Later books there are praises to Aaron's (a) family as well.
The Tanakh praises some chosen ones like David (a) while condemning others like Talut (a) and Sulaiman (a).
It also tells tall tales of slavery being allowed by God from the get go like Ibrahim's (a) slave and later slaves are allowed as well. This is to do with also corrupting the clear Prophecy in Ismail (a) and chosen king to be followed up by Twelve princes (who would become Kings) in his offspring.
But from the get go, we see Torah talks about exalted ones Adam (a) wants to be of. These exalted ones and tree of life, to me, are obviously the ones alluded to by "the one who you will send". That means, there will be a time, when knowledge of Prophets (a) is spread and their knot on tongues opened.
So according to Torah, there is a universal one to be sent who will relieve the knot on all tongues of Prophets including his. Now, really Musa (a) was looking all the way to the Mahdi (a). But God said, prepare your people now and make them come to your knowledge through Aaron (a). If there is a knot on your tongue because of the ignorant propaganda, he can teach them the secret knowledge that won't be effected by that propaganda.
Thus Aaron (a) was more conveying in tongue and more eloquent, only due, to the situation he was in. Moses (a) being the founder, had to speak things simply.
Now, Allah (swt) wants knowledge to spread. And things are now reaching a climax.
Quran says Mohammad (s) is sent to all mankind, present and future to come. His Ahlulbayt (a) are also sent to all mankind.
Although Musa's (a) prayer was pertaining to the Mahdi (a), Allah (swt) guaranteed the relief of the knot of the Tongue with Imam Mohammad Al-Baqir (a). That at that point, the knowledge, will no longer be hidden and it will be spread.
If you see hadiths of Imam Mohammad Al-Baqir (a), he heavily taught about the light of God. He swears, that the Imam is the light of God in heavens and earth, and that the Imams are the light revealed with Mohammad (s), and that the light is brighter in a believer's heart than sun on midday.
This is showing there is more to the leader that meets the eye. Musa (a) couldn't say "I am the light of the world", or people will think he wanted to be worshiped. Same with Mohammad (s), he said to Ali (a), were it not for people doing to Isa (a) what they did, he would have spoken words about Ali (a) that would have truly exalted him.
However, Imam Mohammad Al-Baqir (a) is very explicit in the station of the Imam. The Ziyarat of Imam Zainal Abideen (A) by Mohamad Al-Baqir (a) is very deep.
Ultimately, the light of Imam (a) is the heart and soul of the next world. He is the stream by which all believers drink from, the rivers are about their reality. It's not real honey, not real whine, not real milk, these are metaphors of a higher reality, the reality of the abundance (Al-Kawthar).
And so the Ahlulbayt (a) were successful to a large extent in bringing people to a very strong middle path about recognizing the light of God and Imams. They don't worship them nor do they neglect their exalted status.
The Imam Mahdi (a) is a final test in Quran, for humanity. They all prepared the way for him. The warnings in Quran that not a city will remain but be destroyed or punished severely by God, are there.
The Mahdi (a) is the universal one sent to all, that means, when he is sent, all humanity will witness him. However, we are lucky in that Mohammad Al-Baqir (a) already cleaved the knowledge and untied the knots on Prophets and Messengers. The knowledge from first and last per ahadith he has spread.
And so the knowledge is there. This is not the issue. The issue is application and preparing for the Mahdi (a). The final test. And going back, it's the final test, because the world is coming to an end and this is the climax test from God. He sent the best of the best (Mohammad (s) and his family (a)) and left one last king from them, and so he is the last of God's chosen. How we respond to him and come to him, is the final test. All good things must come to an end, and this trial is coming to an end.