One Word: LEARNING CURVE!!!
OK, so that's two.
Comparing Logic to GarageBand is like comparing Shake to iMovie. With all available windows open in Logic, it's enough to send Expose into a nervous breakdown.
It seems to me that SoundTrack -- which I have neither seen nor used -- may offer more beef in terms of looping than GB (more loops, more instruments, more options for interfacing with FCP, etc.). But GB is targeted to us home folks that want to throw our own music together, using ANY or ALL of the following tools:
1) Built-in Loops
2) Internal Audio from external MIDI
3) External Audio that's MIDI-driven (ie, from GB or other App or ext. sequencer)
4) External Audio from direct sources (guitar, voice, a well-mic'd Triangle, etc.)
In all, very very versatile.
Some of the limitations include no notation (which I can certainly live without) and I'm not sure to what extent MIDI events (including non-instrument triggering events, such as patch changes, volume/pitch/modulation/etc) can be either edited or filtered.
But with the help of other programs (I mentioned
Intuem in another thread, which is only $79), you can still get all the tools you need for a tight budget.