Yes, that's my guess -- the 20/40 games and up work better with $20 chips rather than using 4 x $5 chips (or 8 chips for the $40 bet). You need fewer chips to order, plus the dealer doesn't have to check each bet to see that the right number of chips were bet / called / raised.

In California, poker WAS the reason for the club, in those days. Whereas, in Nevada, the $5 chip was standard for all the other table games. Most poker rooms were just an "amenity" in the Nevada casinos. Not their main reason for gambling.