Dust collection on the ceiling allows you more freedom to move things around the shop. As you add machines, other machines need to move. Moving a hole in the floor a few inches is difficult. Shops evolve over time, they aren't static. Locking machines in place makes it hard to change.
Problem is, it has to overcome gravity to pull the stuff up. Means a bigger unit and being real careful not to leave a gate open where it can suck easy air.