It makes the most sense having the PSU on the bottom.
No dangling wires, top mounted PSU's takes away space for radiators, Design (in my opinion). COG. The big power cables closer to the table (so increases the easy of access on USB's and other cables).
Joedaman: I see plenty mid tower cases with bottom mounted PSU's. Though mostly the newer models from respected companies.
Like my own NZXT H440 and cases like this one.