Unfortunately, the genes developed for resistance are still present in a certain portion of the pool... so that when antibiotics are used it only takes 1 to multiply exponentially and destroy the resistance. Some of the genes cost little extra to be selected against... and even when an expensive group are selected against, it can split up into more than one survivor and then the combination will be selected for again.

Like old code... most of those once useful genes don't ever fully go away.