There may be other reasons I'm forgetting, but here are some:
Reduce chance of warping. Note issues I ended up having with CPU vent.
Using 1-2 front case fans the GPU vent becomes entry or exit point for air (ie. size doesn't need to match size or number of GPU fans). The tiny GPU vans aren't very strong, so the goal is to keep the air temp inside the case cool.
With the L5, I wanted the least amount of vents as possible, and not the most.
I know there are some who like the idea of replacing the stock GPU fans with 80 mm fans. I'm not totally convinced this is a great option. Half the fan doesn't even cover the heatsink. More testing needs to be done. There's so much room down there, I would like to see a fanless heatsink developed just for this purpose.
Come to realize there doesn't seem to be a picture anywhere in this thread or on your website what the bottom looks like. Could you share a pic?