The reason--the only reason--I'm seriously considering upgrading from 10.6 is that new programs increasingly don't support it anymore. I download a simple little helper app and it says "The program requires OS X 10.8 or higher."
My best guess is that programmers build with the latest libraries, and the latest libraries require the latest OS version. If the dev is running on the latest version, it never crosses his mind to do otherwise.
I did the same thing recently, and I've hated it since. 10.7 removed Rosetta, broke "Save As", and annoyified the Save dialog.
But there were just too many pieces of software that wouldn't run, and unpatched security holes didn't seem like a good idea either.
To add insult to injury, Apple doesn't allow you to virtualize non-server versions of 10.6. You can, thankfully, hack VMWare Workstation and keep running your previous machine's image that way, but it's shitty that you have to jump through those hoops. It seems geared specifically towards keeping people from continuing to use their old apps.
I'd like to keep 10.6 support in the open source app I help out with but there are so many improvements in 10.7 and 10.8 from a developer point of view that the latest release will be the last to support 10.6 (and 32-bit macs). By improvements I mean genuine time saving features like base localisation, not "ooh shiny dot syntax!".
My best guess is that programmers build with the latest libraries, and the latest libraries require the latest OS version. If the dev is running on the latest version, it never crosses his mind to do otherwise.