Ease of use. The idea of the cartridge is that you use it once and throw it away. A full vaporizer requires cleaning + can be very expensive. At the worst a new vape pen is $20-$40. It also fits in your pocket easily and looks like an e-cig / regular vape, many full THC vaporizers do not meet these criteria.

I actually have an expensive portable vaporizer, but much preferred using vape pens + carts.