Because the PS4 brand and userbase is greater in the West, plus the MHW presentation in conjunction with the PS4 is more conducive to mass TV advertisement and sophisticated marketing placement unlike the 3DS.
I have no doubt it will sell big in its first month but this will be a heavily front-loaded game, IMO, because outside of maybe Elder Scrolls Online PvE games aren't as widely consumed on consoles as opposed to your typical PvP fanfare spearheaded by first-person shooters, plus when early adopters realize how much grinding is involved a lot of them will gradually phase out.
My final guess is that MHW will be the best-selling MH to date and the series will taper-off significantly in the West after it.