I think the difference between the shows and cartoons that are getting renovated for the new generation and Animorphs is that all the other cartoons have a considerable fanbase and most importantly, tons of merchandise. As popular as Animorphs was 10 years ago, they didn't have any kind of merchandising, and there isn't a big fanbase anymore. It's too risky of an investment, and I can understand why producers are apprehensive about it.
What they should really do, is get an animated series going on Cartoon Network. That way, if it turns out to be popular, they would have more of a chance to get producers interested in a movie.