Cultural brandnames. Carnival games sold well simply that so many people in the mainstream understand the basis. Carnival Games as a name is the basis of the party genre description. Thus people see it and they refer back to things like Wii Play and Mario Party 8 and think it similar. Thus the mainstream buys it as if they were like "sequels".
Same reason for something like Smartypants with the trainer games. Games like High School Musical sell cause its a cultural phenomenon. That would have sold well on any system it was on.
They are only underhyped to the core gamer who understand what it is. But to the mainstream who aren't surfing the web 24/7 make most of their buys by hype, friends and family, and what they see and read on the box at the store.
