It probably didn't sell well--first it was bundled with Final Cut Pro, and then GarageBand was based on it.
I'm sure a future GarageBand 2 could fill Soundtrack's shoes, or they could just leave it as a component of Final Cut Pro. GB generally does MORE than soundtrack (software loops, etc.) But at present, Soundtrack DOES still have important features. Features for making... soundtracks 🙂 Unlike GB, Soundtrack has a video track, complete with marker support, for composing music to fit the footage or animation.
Soundtrack is probably my favorite Apple app ever--although I could switch to GB just fine I'm sure.
Also, Soundtrack comes with many more loops than GB--and they're not software loops, they're all real recordings. A great collection. Do the GB Jam Packs include all of them? I've always assumed there was SOME overlap at least.
(And vice versa... if I have Soundtrack, how much would I gain by getting the Jam Packs? After all, the Soundtrack loops I already have should work fine in GarageBand--so I already have a Jam Pack of sorts.)