There seem to be factors other than the size of the backlog that play into when a case is docketed for.
Illegal aliens, while still part of "The People", are not entitled to as much due process as are citizens.
Deportation hearings are not the roadblocks - locating the illegal aliens in order to bring them before the court, holding on to them until they can be sent back to where they came from, and then actually deporting them are the real issues that need to be resolved. And even if the first two could be resolved, dropping them off the back of some transport plane (with parachutes supplied at no cost) is pretty much frowned on.*
* - mentioned because prior deportation efforts were stymied when planes full of deportees were denied landing rights. It helps to think ahead and plan how to deal with that if it happens again.