I have a theory (nothing scientific) that with a franchise as big as Resident Evil, the previous game's quality/reception has a sizable impact on the sales of the next title. Resident Evil had a pretty solid track record prior to RE6 which meant that many gamers (myself included) were buying the new Resident Evil simply game because they liked the series.
That theory would explain why Resident Evil 5 is the highest selling title of the series, the game itself is good and it was helped by the fact that the previous game was outstanding. On the other hand, Resident Evil 6 damaged the brand name and left a bad aftertaste which then held back Resident Evil VII even though the game received a very positive reception.