What changed in World that made the series so popular in the West?
Quality graphics, good online multiplayer non reliant in local coop for the first time, released in both consoles and PC, less grinding nature, more accessible encounters, much less boring to play single player
And good marketing and advertising, easier to get when you have a good partnership with Sony
But to be fair, before asking why World become popular in West we need to ask why Monster Hunter was even popular in Japan in first place. You can pretty much say the game was a flop everywhere but Japan, so it's not like MH wasn't only popular in West it wasn't popular anywhere besides Japan
I guess the very high number of handheld users in urban areas along with the tribal/Social mindset where you need to play whatever your friends are playing mitigated the lack of a good single player campaign, as such MH was before anything a social game
"World" isn't really a social game, at least not in the same way as previous installments. The lack of sales in Japan (compared to other entries) reflects that.
Different selling points-> Different advertising for a different kind of customers -> Good Reviews and World of Mouth -> Good Sales