Because HTC doesn't put any value in multicodec support. And it's not just high profile that's the issue -- the Sensation can't play mkv natively at all. Nor can it play Divx. It can play mp4 files like the Evo and has Xvid support, but as a video player, it pales in comparison to the S2. I'm not sure if rockplayer or moboplayer can play 720p mkv files on the Sensation.
I actually almost didn't get the Evo because of the lack of codec support. But the fact that I could at least play SD avi files with Rockplayer was the reason I jumped from WinMo to Android. Seriously, if Rockplayer didn't exist last year, I probably would have passed on the Evo.
BTW, I was able to play a High@L4.1 1080p clip (encoded at 15mbps!) on the Galaxy Tab smoothly. But High@L5.1 was a black screen. I transmuxed the clip to mp4, but it still wouldn't play, but I don't know if it was an issue with the file or the device.
And oh yeah -- if the S2 doesn't have 4G (which I find highly unlikely), that's a deal breaker.
How could something have xvid support, but not xvid support.. as mpeg 4 part 2, they are birds of a feather...

i'm a bit fuzzy on snapdragon's abilities quite frankly, so maybe acceleration is possible there. I know that the tegra 2 chipset has a nasty bug/limitation which prevents high profile h.264 from playing correctly. and I think i was mistaken in my assumption that tegra 2 was somehow incorporated into the 3vo. if it's not there, that means this is probably just codec support issue, like cobra mentions and like virtually all android devices that lack the gpu acceleration. But this isn't a problem of snapdragon per se, but of lousy codec api framework of Android. Google could easily build and optimize codec support into Android to allow others to hook into gpu, or at least make it easier for developers to do so. But there's no money in it. Still... I'm befuddled as to how Samsung pulls this off where others do not. I wonder, Is it they have the tools since its their own SoC?
The end result of this perversion (as a present case in point) is I have an xvid file that skips in moboplayer on a 710mhz (OC) Qualcomm processor whereas in Windows Mobile 5, I am able to play it fine. Something isn't def. not right in this picture.