I don't think anyone really knows. They certainly haven't been in a huge rush to do it before now. I believe the difference is that Blackberries are/were supposed to be for business execs and the like so RIM didn't see the need to add flash capabilities to their devices when the memory capacity could be for something much more useful than a flash player.
It would be nice if they would decide to make this a standard feature on all BB devices, but nice doesn't seem to get RIM in any hurry at all.
Posted from my CrackBerry at wapforums.crackberry.com