Why I never considered one Lizzie until they went to the solid back wall and a real RV style solid door. That was a terrible design and they all leaked.
The newer ones like I have, the side walls are higher but the top is lower so you can actually use it (toilet, fridge and dinette) with the top down. Makes it nice for a snack on the road or using the toilet. You still have to watch your head of course but you can get in unlike the older ones where you had to raise the top to swing the top section up to open the door to get in.
Why they aren't worth much used. If they has not redesigned it, I would have probably bought a Northstar because I require a pop up. a Hardside don't work for my uses.