History man, history.
Where is IE for the mac? It's gone.
Where is Microsoft's media player for the mac? Piece of outdated junk.
Outside of Office for the Mac and some other minor things, MS is 100% invested in the MS stack - it doesn't make business sense for them to develop for other platforms - it's a broken system when it comes to open standards.
If they spend enough money coming out with version 1.0 for other platforms... and if they spend enough money buying downloads of it so people will use it during the Olympics... then maybe it will take off. Then when it's a "standard," what's the incentive for investing in a linux or mac version at all? Heck look at IE after version 6.
I'm not saying that it doesn't have technical advantages over flash and that it isn't nice to have readable content for search engines, I'm just saying - look at where MS's money goes. It doesn't make any sense for them to be more open than they absolutely have to be - that's Microsoft's business.