Fanboyism 90% of the time. People like to brag about there exclusives as a mark of superiority. It really burns when after all of that bragging and dissing the other systems features and capabilities, an improved version comes out on another system.
I generally don't see that as much from the Nintendo side though. No one really cared when No More Heroes, Resident Evil4 , the Resident Evil Chronicles games or Little King Story got ports to other platforms.