Exactly! I think that fact is key to many of our other observations.
Casinos know blackjack players are blackjack players, and they don't expect BJ players to get up and start playing roulette or slots. But they see SP21 as a carnival game, not something with a history and a following like BJ. They also surely "read someplace" that the house edge on SP21 is 0.44%, as opposed to 2-6% like the other carnival games.
Now put that all together, and to the casino SP21 is an unprofitable distraction to their carnival game players who they'd rather see playing Let It Ride and Caribbean Stud. They think a SP21 player is a sucker who's getting off cheap with a 0.44% edge, and this is why the SP21 tables are perpetually few, full, and fermé. But they don't realize just how bad a typical SP21 player's play is and that he's giving away far more than any BJ player, as well as playing a sidebet with a 3% edge.