A different soil type. The soil in the west is more alkaline then what is found in the eastern regions of the U.S.
Kentucky Blugrass, Poa pratensis, is found in Colorado (and in all 50 states). It is not tolerant of drought or heat, so it is less common in the Deep South southwest than in the northeast, midwest, and northwest.
because they don t have hillbilly piss in those other states