Greed, IMO. Have seen different posts about some BnM's that have both a room and the machines, while some have just the machines or just the live room.
Casinos know that a lot of players are too timid (and other human traits) to sit at a live table, but they can still make their rake off a machine. It satisfies their greed and gives non-live room players an option instead of just the video poker slots.
The more poker players there are, the more space a poker room demands for gaming floor space. On a 'per sq. ft. ROI', the machines are far more profitable to a BnM operator. As noted before, it also eliminates the need for a labor intensive live room.