I should note that a card with Shader Model 3 can be had for as little as $25. For less than half the cost of a game, you can get a SM3 card. For that matter, for $100 you can get a Radeon 4850, which has SM4 and is fast as hell. Quality graphics are not expensive these days, and thus gamers really don't have room to complain and not get them.
As for market share, according to Steam, who has some of the best numbers on things (since they take data form actual systems and they are systems used for games) 29% of systems have a DX10 card, and run Vista or 7 and thus support SM4. 59% of system have a DX10/SM4 card, even if they don't have an OS for it. 82% of systems have a DX9c/SM3 or better card. 89% of systems have a DX9/SM2 or better card.
So while people do have DX8 or lower cards, they are by far the minority. As such it doesn't make much sense for game developers to support that, especally since SM1.1 is extremely limited in comparison to the newer ones.
As for Macs, well you should be used to the fact that it is a minority platform, and as such doesn't get so much support. That is just life. If you want to be a computer gamer, you really need to have a Windows partition. If you aren't eilling to boot to Windows you need to accept that your selection will be very limited.
That's just life on a minority OS. Developers have to weigh cost vs gain for porting, and they often decide it isn't worth it. They'd have to hire on a number of new developers and testers, use differnt tools and so on and the potential marketshare increase isn't that much.
