That's a good question.

I have no credentials or background to know the answer, but I wonder if this is something where the use of antibiotics decimates most the bacteria population so that it's easier for the resistant ones to thrive, as opposed to cohabitate and possibly be overtaken by the non-resistant ones. I'm thinking of it like Conway's Game of Life where a cell dies if surrounded by more than 3 cells. With antibiotics, you limit the number of living cells around and are more likely to keep others living.

Or maybe the non-resistant ones would have another trait that make them thrive more than the resistant ones if both are mixed?