Also the wide support for UE4 in general might have been a reason. For your own engine you probably would have to, for example, implement a lot of features yourself rather than just buying stable and reliable 3rd party plugins like Substance, Enlighten and so forth. Also the giant userbase, especially among employees prior to joining, makes away with long learning periods. Also there are a lot of systems in there that seem to speed up development a lot (bluepring, the material editor and the easy drag and drop UI for basically any kind of file format (aside from gif as it seems).
About gameplay: iam totally in for modernizing the whole thing. We had so many iterations of the same game now, iam really psyched to hear about what they are going to do.