Any business must be profitable and a lot of these smaller exchanges are not getting enough customers to make it sustainable, so they just go the exit scam route to recover their losses.

Other sites do this as a long con, because they know the exchange will not succeed, but they do it anyway, because they know they can get a lot of money at the end, if enough people join their service. They just monitor the deposits and when enough people deposit a large amount of money, they cut their losses and run.