[marssaxman]

They had to support HFS+ in order to convince existing users to upgrade. The first five releases of OS X had something called "Classic mode" which was basically an instance of OS 9 running as a process, so that people could continue using their pre-OS X apps. It took six years before enough developers had upgraded that Apple could stop supporting Classic. And Classic, of course, was all about HFS+. So Apple could either backport this hypothetical unix-style filesystem to OS 9, do something kludgerous with single-machine file-sharing, require all users to partition their drives and keep everything they wanted to use in Classic on the HFS+ partition (a tough sell!), or just use HFS+ and get on with it. They wanted to stay afloat, so they stuck with HFS+, and here we are today.