And because the hardwares are different too, so they can not run the same softwares.
That's no excuse though, they've had several months to come up with a 4.3.x build for the CDMA iPhone, it just seems like they have chosen not to. iOS 5 beta has CDMA builds though, so I think they've just decided to completely skip us CDMA users on 4.3.x and the next major upgrade we will see is iOS 5. Hopefully, they at least manage to get THAT to us at the same time the GSM users get theirs.
I've dealt with Verizon for long enough to know that they ALWAYS tend to be behind on OSes for their devices, though. They are so anal with their beta testing that almost nothing is ever good enough for them. To be honest, I am not getting the "Apple Experience" with my VZW iPhone that I thought I would; as I though we would at least be on parity with the GSM users as far as OS goes.