I think the main reason for C++ in UE4 is simple, it gives unrestricted access. If you want a feature, it is just on your resources and skill to implement it. With most engines, if you want a feature not explicitly thought of you either had to buy a Source Code license or were SOL.
One of the biggest complaints I heard about UE3 and previous versions was UnrealScript. While personally I liked it, companies hated it. The reason was it made for a very small pool of people to hire from that had experience in it and I watched most programmers just fight it and complain. By the time they had learned UnrealScript well enough to use it they had allready designed and implemented the main architecture for the game and there was no turning back to redo it. Now, companies can just hire C++ programmers. Much wider pool to find engineers with experience.