The thing about modding is, simply put, you can only do so many retextures and script changes until it gets boring. When you're limited to what you can 'add-in' you limit modders replayability. Same thing happened with FC, was nice and hot when the game launched same old weapon enhancements, overall retextures and unlockables. Basically, GIMP a few things and play in the scripts for a bit. Once people played with that and got the game how they liked and maybe done a few requests, that's it. Take TES for example now, you have the ability to throw in models, customize map layouts, sounds, all the goodies and the community is still quite strong. Hell, not too long ago I was working with some folks on a complete overhaul of that game to fit around the Inheritance Cycle series. All new NPCs, models, voices and a whole new world. New Vegas is still going too last I noticed.
So when you limit what can be modified you limit yourself to a small and brief community that WILL move on to other things.