A mixture of games, a mixture of rules, an ebb and flow of customers with ever changing whims and preferences.
I think Doc is right. They may not even notice the table is 6:5, or even think about the difference between 6:5 and 3:2. The world if full of dumb or lazy gamblers.
The evidence of this is every casino anywhere, who has installed 6:5 tables, and gets plenty of action on them. If the world was full of smart gamblers they would be deserted.
I personally been at single deck or spanish 21 tables...and despite all the signage and different layouts. People often sit down buy in for chips and then notice it's a different game, only when the cards are dealt, or sometimes not even then. I actually had people ask me at single deck...why are the cards face down. I explain it's single deck you pick them up and play them. They almost always use two hands and get yelled at by the dealer. They play for two hands look puzzled and leave usually.
I dunno why they deal the cards face down.
There's places that deal SD/DD face up. Same game minus the confusion by new players, mishandling/damaging of cards by drunk players, and all kinds of cheating scams involving switching/marked cards (it's so easy to mark cards in a face down game it's not even funny), and it's harder to cap/pinch a bet on a face up game.
And the game typically moves a tad faster if it's face up for experienced players, and a LOT faster if there's new players at the table.
Also, there's less complaints about some noob hitting their hard 19/20 by mistake (which generally gets resolved in the player's favor at least the first time)
If someone wants to learn about blackjack or gambling in general, today there are plenty of resources available. WoO being a prime example. I also think the American Casino Guide has some good plain talk explanations. ACG is available at my local library.
But some people don't want to be bothered with that. That is their choice, of course.
Some people like to play slots. I like to play Video Poker. I do believe that the high house advantage games help support the availability of the lower house advantage games, promotions, etc.
In my youth, I liked the Strat because it had a handful of ~100% VP games, and the rooms were inexpensive.
Well, learning is old-fashioned. The new way is do what someone else says to do (social-networking at its heart). keep the dumb being dumb, and always complain about the truth, "Its not right for ME". And you wonder why people are dumb... they learn from knowledge-gate-keepers. Gossip is a terrible education sez I.