Laziness. No, really. They made the Catalyst to solve the problem of their thrall races wiping themselves out in robot wars, because they were more interested in just soaking up the tribute than in actually solving the problem themselves. No wonder the Catalyst decided they were part of the problem. Even now, the Leviathans aren't mad about the Reapers trying to kill them, they're mad because tribute does not flow from a dead race.
Makes me think that the Leviathans themselves might be the next big bad. They were only ever allies of convenience.