It's because they are inexpensive, require zero maintenance, zero mess, zero additional equipment needed to maintain and use it. You don't have to understand anything about how it works or is put together. You get to have new colors and styles frequently, you naturally keep up with the trends. If you lose it or break it, it doesn't matter. There's no compatibility between the cartridge and the vape to think about.

Basically, because they are as easy as possible and stay trendy by their revolving door nature.