Not to be that guy, but I think your history is a bit off. There had been 6 mainstream Final Fantasy games before Pokémon released and thousands of other huge RPGs. Rune Factory and Harvest Moon were massive before Starview. Smash Bros is a footnote in fighting game history. There have been a steady stream of decent metrovania’s since Metroid and Castlevania originally released. Roguelikes have been a staple for decades and were very mainstream before Dead Cells etc.

Regardless, the simple truth about SHMUPs is that high score chasing is an acquired taste. A lot of people are turned off by the idea of doing the same thing over and over again with the sole aim of doing it better. A lot of people want more variety that that in their entertainment, get frustrated with failure and want to turn off and enjoy more passive experiences in their leisure time.