Would guess because being at a bar is a more fluid experience than at a meal where you pick a table and stay there. Also at a bar, you tend to order at the bar and then go back to your table, so the bartender doesn't necessarily know where you are when you aren't ordering something.
It seems pretty common to forget to grab your card at the end of the night. I can imagine how easy it would be to "forget" to pay, especially if the register didn't have any of your info upfront.