Indeed ...I even dare claiming the engine is not responsible at all.
What's wrong with UnrealEngine3?
Source engine is a regress or at least a stagnancy. If you want something real fresh and new that can handle massively multiplayer matches with an awesome graphic you are optimally with CryEngine2 if you will so.
But my choice would be UnrealEngine4 because of the editor I knew from previous versions. In my opinion there is nothing comparable. I just love it.
What do you think about it?
Then UT3 would be much more popular.