The reason is of course clear: rc4 and rc6 have shader conflict, so one of these must go; however, I’m convinced the wrong one was deleted.
rc6 is effectively the same map you can play in UrT 4.2 — it’s a prettified and bit more closed down version of 4.1 Kingdom (middle roof access more difficult from blue side, for one thing). rc4, on the other hand, has a completely unique layout, with various added rooms and passageways; its openness makes it the version for the kind of huge massacre TDM we have here. (I can see why the layout was closed down again in later versions, it’s absolutely not suitable for competitive play.)