The short answer: because it’s a pretty crap plug.
If you had a plug that was more of a hassle to get out of the drain, it would probably hold water in a lot better too.
But, well. Those are often made in rubber-ey materials with some plastic mixed in. Those can dry out (yep, even if they are often submerged in water) and stop being good at their job. It sounds like it’s time to consider replacing yours.