Is it a fad that they've become so popular? Maybe. It would seem a lot of folks buy them as "essential" gear without thinking about their needs or why they're buying on. I'm over ground tents. I move a lot in the rig, and after a lot of evaluation, the RTT meets my needs a lot more than a ground tent.
There are some interesting perspectives here.
This for example, in no way shape or form looks like an easier more compact solution to a RTT to me.
Then again I've been using backpacking style tents the whole time. I was already used to not standing up in a tent, and setting up cots and chairs and ovens. I mean maybe that's a thing for some folks. So the transition to having a RTT is a simple one.
To each their own and how they want to camp. All that matters is what makes you most comfortable. Not what the other guy is doing.