Having teeth that aren't straight can actually have implications on the functionality of the mouth in some cases. That's one reason.
Also, who wants to have crooked teeth when they're older? I know I'm happy I got braces when I was younger. That's two.
More types of braces available now, which can be discreet and comfortable - so much more appealing to younger and older patients.
And braces bring in a good amount of money for the dentist. That's a definite three.