Block RAM in FPGAs is very limited and adding something like a DDR3 controller is a hassle - of the dev boards I have lying around, they have between 48KB and 200KB of block RAM. The actual CPU logic is probably <5% of those chips. I have a board with a 16-bit, 16MB PSRAM interface, but then the memory interface is shared between the video generation and the CPU. Going with 4-bit colors or something seems like it would be more in-line with the design philosophy (but it was targeted to be emulated on a modern PC, where memory and color depth are largely free).
