> why isnt now such a jam?
Well "retro" literally is defined as "imitative of a style." For jams that mandate authentic ROMs you'd be better off joining a jam dedicated to a -- or set of -- system(s), like the jam you mentioned. If you'd like one specifically for the NES, there is the NES competition over at homebrew forum at NESdev.org (actually it looks they they are hosting it here on itch this year: https://itch.io/jam/nesdev-2022) .