Is it the EU fault that everything had to have a backstory? Is it the success of the 90's Eu that the prequels were even made?

I would charge that both are true.

Characters like Boba Fett needed a back story because people did not want any sort of mystery and they wanted all the blanks filled in.

The EU even infected Lucas thinking as far as i can tell. Before the 90's he insisted the EU did not exist and was separate from his universe. Flash forward he uses the EU for source material to make the prequels.