I would guess it's due mainly to Mike's touring schedule (and the apparent fact that they seemingly *rushed* a second season into production to strike while it was hopefully still hot), and also that the huge nostalgia factor that is leading people to watch this awful show in the first place has more to do with 80s nostalgia than people from the BB's era. True, the BB's via "Kokomo" and being on the original run of the show are part of the 80s nostalgia, but I'm guessing people who grew up in the 80s are inclined to be into "New Kids on the Block" and other stuff that was more part of youth culture in the 80s (witness the insane success of the new "NES Classic Edition" videogame console).
That crossed my mind too, yet I'd be pretty surprised if Mike wouldn't have made some Hurclean efforts to switch a few shows around if the opportunity arose for a Fuller House appearance.