At this point, 10.8 is out, so developing on 10.5 means you need to test on 4 different versions, each with their own quirks, just for Mac OS X. There are lots of little things that you discover break if you develop on the latest OS, and then test on the older one (or vice versa neither forwards nor backwards compatibility are perfect). Another OS version just requires more development time and more testing resources. It doesn't have to be features that are required.
