For one, “my bad,” roles off the tongue much easier than, “my mistake,” does (even if, in practice, it saves very little time). It might not be quicker to say, but it is easier.

In addition, it can convey a certain level of informality or perhaps a lack of severity that you wouldn’t necessarily get by using the more traditional, grammatically correct, alternative.

Finally, it has a certain syllabic flow to it that, while hard to quantify, makes it quite catchy and repeatable. I’ll freely admit that there’s a certain je ne sais quoi factor that, while hard to quantify, is definitely there.

Personally I’m not sure what’s so horrible about using, “bad” as a noun instead of an adjective on occasion. But we all have to choose for ourselves which ditches are worth dying in*. By all means, don’t let me stop you from making this one yours.

*I just ended a sentence in a preposition, oh no!