I pulled this off the internet:

"As you have no doubt heard, adding more card decks to the game of Blackjack increases the house edge. In fact, most players will tell you that basic strategy affords a player the best odds when the game is played with just one deck. Interestingly enough, a number of casinos have introduced the single-deck game lately, but as you may expect, there is a catch. In fact, the new game offers players hands down the worst odds of winning of all the blackjack games.

While casinos lure the player in with the single deck, many players don't notice the seemingly slight change of rules in these games. Look carefully however, and you will see that an most of the new single-deck tables, the Natural Blackjack payout has been reduced from 3:2 down to 6:5. (This is why these games are also known as "6:5 Blackjack".) Look at how this affects your winnings for a $20 bet:

Payout Ratio: 3:2 x Bet Amount $20 = Win $30
Payout Ratio: 6:5 x Bet Amount $20 = Win $24

In the game of Blackjack, the house edge is derived from the fact that the player busts first, and loses the bet even if the house subsequently busts as well. To partially offset this, the player is given various "perks", such as high payout for Natural Blackjack, the ability to Split or Double Down. By reducing the payout for Natural Blackjack, the casino gains a much bigger advantage over the player than they surrender by having only a single deck of cards in play.

In fact, the player is likely to lose between 1% - 1.5% more at this new single-deck game than at more traditional forms of Blackjack. This means that if you are playing $20 hands, you will lose roughly $20 more for every 80 hands played.

In promoting this game, casinos are taking advantage of the fact that a large number of players have heard that single-deck games offer them an advantage (however slight), but few realize what they give up by accepting smaller payouts for Natural Blackjack.