Lots of reasons but keep in mind it's best for just two people.Great for wet ground, keeping away from critters and insects, you can camp anywhere (super mobile), when you get tired you can stop, no ground stakes needed. Sets up in about 10 min, it's more bed like with sheets, blankets, comforters, etc (better sleep). More for overloading, multi-day travel, avoiding hotel fees than for extended camping or family camping.

Personally I hate setting up a tent, even the easy set up canvas tents, a rooftop still takes work but for some reason it feels different. Hanging out in a tent especially during hot days sucks so I get out and do stuff. I also don't like extended camping in one spot or leaving all my stuff at a camp site unless people are there. With the tent on the roof it's easy to pull out a canopy, chairs and table, cooking stuff and put it back and be on your way.

Drawbacks are the ladder, can't stand up in it but you can get a room that zips on the bottom of the tent and allows you to stand on the ground if you want, the size and weight on top of your vehicle. http://www.arbusa.com/uploads/PDF/pressReleases/touring_series_3_retail.pdf

Another type of tent that is similarly hooked to a vehicle but is a ground tent like a canvas tent is an OZ tent. Lots of people like these because you can leave them behind but still have them close to the vehicle for access to your stuff. They have lots of different options to chose from http://www.oztent.us/products/tent-range/oztent-range.

Ultimately it comes down to type of camping, number of people and cost. A rooftop tent is a couple of grand but much less than an off-road trailer.