Basically its different technology. Disposable products are a one piece design and are made to throw away after a certain number of puffs which varies between 200-500 depending on the size.
A rechargeable product is normally a 2 x piece design which comprises of a rechargeable battery and a cartridge which contains the E Liquid. Most cartridges are designed to last about the same as a pack of cigarettes and are then disposed of. The user then purchases refill cartridges often sold in a pack of 5.
Rechargeable E Cigarettes are considerably less expansive if used over a period of time.