In some ways that quirk exposes an anomaly in England's selction - that arguably the better keeper and certainly the better batsman doesn't get the gloves - but England have improved massively over recent years because of consistent selection. Buttler has never deserved being dropped and therefore Bairstow came in when a different vacancy arose.
Long term, its one they'll have to look at. If Bairstow isn't quite making it as a btsman but scoring more heavily than Buttler, say...