It is obvious when you think about it...

I've heard from several people things like "The Back to the Future movies say we'll have flying cars by 2015. That's only 2 years away, where is my flying car, hoverboard, self drying clothes and all the other stuff they showed?"

I guess it's my fault we don't have them since I'm working on giant robot freezers, right?

But, it occurred what the problem is.

We're in the Biff version of reality, not the Doc Brown one. Somewhere in Vegas, Atlantic City or Connecticut is Biff with that guide to betting he got in 1955.

This explains so much!
Now, we just have to wait for Marty and Doc Brown to get the book back in 1955, and the universe will reform around us with flying cars and all!

I hope they do it soon...