Which is fine. But shouldn't you _expect_ being left out by now? XP came out in 2001 (hazy memory, quoting Wikipedia). OS X (10.0, Cheetah) came out the same year. What's the minimum requirement for the Cocoa app?
Well, a quick lookup says 10.6, which is from 2009. Which, incidently, is the same year Windows 7 came out. So Sourcetree needs an OS that is no more than 3.x years old..
Bugger?