Why is PHPosxom no longer being developed?
The reasons for ceasing development on PHPosxom were varied, but the primary one is that at the rate things were going it would eventually re-invent the fine WordPress platform, which seemed a silly thing to do.
As I say, the code is still available, and whoever would like to is welcome to begin their own branch.
Thanks to everyone who tried out PHPosxom!
Matt Gemmell (of Irate Scotsman and Scotland Software fame) created a fork of PHPosxom called Thistle (the name for which I will immodestly point out I came up with ;), added features, and really hammered the code into better shape. However, like me, he has left the *osxom projects behind for different blogging software waters.