Speculating.... but there appear to be two fairly distinct pathways towards "resistance", and they do not meet.  If resistance is defined as some kind of bee characteristic that influences mite levels so that colonies have a better chance of survival.  

One pathway comes from modification of colony level traits, such as timing and amount of worker and drone brood production, swarming, possibly others.  It likely arises through natural selection, and produces commercially useless small colonies that limp along with fairly high levels of mites.  Selection for this moderate level of resistance may actually benefit from keeping mites at moderate levels (as opposed to very low levels) because the result is smallish colonies where mites do not get extremely high.  So this selection may actually have an optimal plateau of mite levels that are too high to allow the populations normally needed for useful commercial production.  If mite levels drop below that optimal level, then colonies might grow to points where mite production escalates and takes out the "resistant" colonies.

The second pathway comes from individual bee behavioral traits that when selected to a high intensity can produce colonies with very low levels of mites and useful populations for commercial beekeeping.  This pathway may be present in incipient forms in some naturally selected populations, but does not appear to be as fixed as is needed to keep mites at levels that colonies do no limp along, surviving, but not thriving.  The only situations that I know of where practitioners using individual bee behaviors have commercially useful colonies that keep mites low is where they select and maintain colonies with highly fixed traits using controlled matings.

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