I have to say it's how complicated it is to care for them. I've had other reptiles and find other reptile species really beautiful, but their care is so simple that I always get bored and sell them. It's different with chameleons - I get to tinker with their cages, lighting, water, supplements, gutload, etc. And it becomes several different hobbies in one, always trying to make things better or more efficient.
It's their look that caught my attention but their level of care that still has it!