Part of the issue is how "the thing" (in this case, gambling) mutates post-legalization (and also via technology). Sports betting was a minor pastime that a small percentage of the public engaged in, mostly just occasionally and usually for minor stakes. Part of that was the inherent friction - you needed to either be in Las Vegas or have an illegal or offshore bookie. And still, you needed to place bets in person or on the telephone. Today it's in your pocket 24x7, every sport available including international, and instead of betting on a game, you can bet every quarter, every player, every play. The degree to which sports gambling has penetrator society has multiplied probably 100x over what existed prior to legalization.

Oh, and sports is just a doorway to full mobile casino gambling (slots, blackjack, video poker, etc), already legal in a few states, growing quickly, which will ultimately be a much larger gambling market.