Bad marketing is a big part of it. Amiga fans will also cite poor management choices in general, though I don't know anything about that.

By the time they were introduced, though, the PC and Mac were already in the market.

The PC was being adopted into offices by then, so it's also possible that the ironclad rep (at the time!) of IBM meant that the PC was already locked into a path to dominance, but that leaves open the question of why the Amiga didn't manage to survive in homes or education.