Maybe the G version model of releases is starting to tire people out? 4G didn't do particularly amazing either.
Yeah i would assume that the case...especially since MHX was already somewhat of a spin of the forth generation games. At this point the fanbase probably is ready for the next big jump.
This is where a potential Switch release would make sense, the more core oriented users would likely jump on a superior MHXX HD Switch version while waiting for MH5.
Same for the western market....