I think it's also more about on the characters for figures / merchs.
RPG has richer characters (both on quantity and development) compared to platformer.
Yeah, maybe if Ristar had more than one game then despite it's cartoon like design it could have been more marketable.
I mean, I have a feeling Ristar was supposed to be Sega's answer to Kirby at the time.
This is why I wish Sega were more intuitive over their IPs. Kirby is older than Ristar but still around.