By supporting the old stuff, they would need to support 32 bit systems. This would degrade overall performance by forcing Apple to spend programming time on old hardware that few people use.
If you think they are not still supporting 32-bit programming in Lion, you're utterly and totally clueless about Lion. Apple hasn't even bothered to offer a 64-bit Carbon Free iTunes for OSX let alone fully eliminate Carbon from their own software. They're too busy being hypocrites and trying to cash in on iPhones to actually be consistent about their own operating system. What ever happened to resolution independence in OSX? We were supposed to get that in Leopard (parts of it were even in Tiger). It's not even going to be in Lion, three "major" versions later. Every move Apple makes now is based on one thing and one thing alone...greed.
If Apple really cared about being the best desktop operating system out there (as in in reality, not in their propaganda), they would have been concentrating on the features they need to stay in the game. Here's just a small list of REAL improvements they could have made by now and won't bother:
* Resolution Independence (finish the darn thing already!)
* Latest OpenGL (not versions from 2+ years ago)
* Keep the darn video card drivers updated or let/pay someone else to do it! Even on the same hardware running the same game, Windows always runs games much faster between the crappy/outdated video drivers and outdated OpenGL and general lack of support for game developers by Apple. Support all the hardware features of the cards as well (it's pathetic that I get full H264 hardware decoding support in Windows on my 2008 MBP, but not in OSX! Even Intel integrated Netbooks have video hardware assist, letting them run HD video that much faster Macs with the same chip set couldn't hope to run in OSX).
* Support USB3 in OSX (at least 3rd parties could offer it more easily even if Apple doesn't give a darn about its customers needs)
* Update or replace the HFS+ File System (it's getting long in the tooth; if not ZFS, roll their own)
* Improve Dual Monitor Support (you cannot even disable the 2nd monitor from software and having to move the mouse to another monitor just to access the application menu is downright STUPID and should have been addressed 8+ years ago. Just being able to have a menu for each screen would have sufficed. Offering docks across both monitors could be useful too. OSX's multiple monitor support is just plain SAD compared to Windows or even Linux).
That is why they are the best revisions. Software developers need to get away from piling on new features and put more effort into making their new software leaner, faster and more reliable as well as less exploitable. The first step in doing this is to cut everything you can out.
And yet Snow Leopard runs
SLOWER than Leopard. So much for your "leaner, faster"
NONSENSE about cutting everything out being the greatest thing ever for Apple to do. It's easy to delete support for older machines in a money grab effort to force upgrades, but a little more difficult to make OSX truly world class. But in a world where a good chunk of the Mac user base are fanatical, they think everything Steve does is perfect. Some of us, though, don't care about Apple or Steve personally and just want a good product.