Well, if he used any other version of the Hadith it would contradict his primary teachings.
How do you know the other version of Hadith is more correct than the one Bahaullah used?
Also you missed the point about ahadith. Bab and Bahaullah predominantly quote Shii sources and Shii doctrine.
Yes, Bahaullah in the Iqan quoted mostly Hadithes by the Shia Imams, but in some other Books, He also quoted hadithes from Muhammad as well.
Do you think, the Mahdi is not supposed to quote any Hadith from Shia Imams? Shia Imams could not have been possibly truly chosen by Allah? How do you know that?
This logic, is like, if a Jew dislikes Quran, because, the Quran says Jesus was the Messiah. Something that the Jews do not accept! So, now if Allah says, Shia Imams were chosen and inspired by Allah, this is is not acceptable in the eyes of a Sunni Muslims, because they see it against their own belief.
Another example, is like some Christian who when they see, Bahai Faith confirms Muhammad and Quran, they don't like Bahai faith, as in their eyes, certainly Muhammad is a false Prophet. I hope you understand what I am trying to say.