I've done a little bit of game development and the thing is if you've already started you don't have a lot of choice it takes a huge amount of work to refactor to another engine and to be honest it's basically the same as just starting again.

Sure you can reuse a lot of the art assets but depending on how far through production you are, they may not actually you've been finalized yet anyway so you've basically got nothing you can reuse. People might have decided that since the changes that were announced (the licensing fee only applying to new versions of the engine) it's worth it for this game and then they can move to another engine for later projects.