From "The Demon Under the Microscope" by Hager--an interesting read on the history of Sulfonamides (the drug that kicks off the "golden age of antibiotics on the chart)--usage was entirely uncontrolled and people did not complete drug regimes. Generally (and I'll probably get this very wrong), that antibiotic worked by blocking uptake of external energy sources.

The drug was tremendously effective and widespread. The book made it sound like all of the strains that did not evolve a solution to that antibiotic were completely wiped out by the evolutionary pressure of the antibiotic.

I'd imagine (just guessing) that even if the antibiotic was stopped, the organisms would not have anything like the evolutionary pressure to change back (and the consequence of getting wiped out by the drug if they did). They probably get along "good enough" or the drift back is slow. In other words, the pressure to change was one of, essentially, absolute survival, while the change back is one of risky convenience.