Maybe the studio execs want a younger Robin, so they use Tim Drake.
I think it's probably the other way around. The producers of B:TAS probably wanted Dick to be younger, but the show was on FOX during a time when the censorship on "children's" shows was pretty strict, so they were required to make Robin 18 years old. (Which is odd, since the Power Rangers were supposed to be high schoolers.) Once they were on the WB with its milder restrictions on content, they were free to feature a younger Robin at last. Obviously they couldn't have Dick age backward, so that meant bringing in a new Robin -- and that let them incorporate Tim from the comics. And meanwhile they took the opportunity to allow Dick to grow up and begin the next phase of his career by becoming Nightwing -- which is the sort of thing you do when you like a character and want to develop him, not when you hate him. We probably would've seen him in more episodes if the series had run longer.