https://www.thormotorcoach.com/outla...oor-plans/29s/
is the link the Class C version you are referring to?
I think they may have heard from potential or existing customers that a side entry might work better, and it might also actually provide more 'length' to the garage, versus if it was turned the other way.
Also, a side entry, which is on the patio/awning side of the rv, might afford some folks a better avenue for entry/exit or usage, versus the rear where you might run into a blockage issue because of a tree, lake, stream, etc. as well as potentially allowing you to back farther into a campground or rv park site.