Because you don't have enough time to make good-looking graphics.
I had a ton of time messing about with 3D on this one, that's what counts in my book :) I did 2D on almost every gamejam for years so this felt different.
Good idea but I just ment that you should join longer game jam and not that you shouldn't make 3D game :)
raytracing update soon! @fishtrone
