You know what's great about srb2? You can endlessly make content for it. No need for an srb3 because why not just make more for srb2? Srb2 is never finished, because there can always be improvements, or more content to add.
Also, why even have an srb3? What would it have that srb2 doesn't? Some might say better physics. Well then name at least one good fan made sonic game with better and more stable physics than srb2. Some might say for level creation. Really? How many people here can 3d model and understand how to format it into a level? Because that's probably what a new engine would require if you decide to go to 3d. And who is willing to learn all the new methods to make simple mods? Srb3 would hinder everything that makes srb2 so great.