Two words, backwards compatibility.You can't just go changing the map format and such without care as then your client is no longer a Quake II client but a 'whatever the hell your mod is called' client. Given the current limits of the Q2 engine, adding such huge changes and still preserving compatibility with 3.20 clients is impossible. Scripting, shaders, etc - how do you expect them to run on 3.20? Granted some engines like EGL already support optional shaders, but one of the main things I noticed about Quake 1, Doom, etc is all the fractured communities from all the different versions which are incompatible with each other.