Basically Universal's target audience was becoming too young to remember the movies (the latest BTTF movie is still 17 years old IIRC), plus part of the ride takes place in 2015 which we're fast approaching and its nothing like that.
Then the ride itself was in need of serious repair, so instead they decided to make a whole new attraction based on the Simpsons.
BTW, I think the Japan version of BTTF is still open, so if you want to ride it, just hop a plane to Osaka!