Creative limitations would be a big reason. I'm sure someone else can explain it better, but being forced into a far smaller scope than you'd usually work in makes you have to consider the impact of every small thing. Its not just a programming thing, people do the same with music, art, any medium.

Its also quite nice as it acts as a levelling of the playing field. The limitations of modern tech are far outside the reach of most solo-devs, but put 100 devs on it, and you could reach it (and therefore create something more technically impressive). With those limitations on, a solo dev can far more conceivably create something technically impressive.

Unless it has real world consequences (eg you can export to a gameboy ROM or something), I don't really care if its based on an actual retro system or a fantasy one, they achieve those same results.