A conventional tumble dryer, regardless of whether vented or condenser, works by pushing hot-air into it joined with the spin effect of the drum.
This implies that the machine needs a heating element which isn't the best energy or money saving techniques for drying your clothes.
If you use a combined washer/dryer, you have a similar requirement for heating, and regardless you need to dispose of the warmth without it disseminating into the room and causing any damp in the process.
A spin dryer works without the requirement for heat as normally it's a vertically mounted drum that spins at really high speeds... driving the damp out of the laundry without any use of heat. This also means it can be much cheaper to run, and to buy. Being top-loading machines they also take up less space.