Oh, please. How many people are paying to run Skype on a system that can't or won't be upgraded to Snow Leopard? Supporting Leopard means that Microsoft can't use APIs released in the last 5 years. They probably have to support x86-32 or PPC processors (which is the reason most people on Leopard are still on Leopard). They have to use relatively ancient tools to compile the packages.
All that, or they can just decide to never, ever upgrade the underlying protocol to handle new security requirements or additional features.
I can't for the life of me figure why MS would want to bother to keep supporting that old code. What's the return on investment for keeping someone's PPC Mac limping along? Or perhaps that's it: they want to make it easy for people to stay on stone age hardware to try and compel Apple to have to support it. Sounds conspiratorial, but I'm hard pressed to think on a non-conspiracy explanation that satisfies Occam's Razor.
Also, Microsoft is killing support for their own WP7, whose last release came out less than a year and a half ago. So much for your assertion, huh? Maybe they've just decided that supporting a 5 year old OS X version has a better business case than Windows Phone 7, which is very likely true.