This question arose in an another discussion about GR. Anyway, why CC must be constant? In GR it defines the geometry, at least without matter present. The geometry, as far as I know, is changed from the Big Bang till our epoch. So assume that there is no matter at all, as what the constant CC can be from the physical point of view? And vice versa, if CC is a function, of time for example, what it can be ? I personally think that this is definitely not a constant but rather a functional which depends on metrics interaction in bi-metric theory or hidden variables a la Prof. t Hooft or something like that. In fact any non-trivial mathematical complexification of GR leads to the terms like this. Technically, GR with non-constant and emergent CC is really more complicated then the ordinary GR but conceptually it must more clearer.