Because people stopped buying the game with free updates and zero renueves for them aka lootboxes or paid cosmetics?
I think it's actually because World doesn't have the legs Monster Hunter games usually do. Especially on Nintendo consoles, they used to sell very well at launch and get more sales over time as friends convinced each other to buy it so they could play together.
You don't really have that with World because there's no local coop aspect to it, meaning word of mouth purchases are much lower than usually and all the sales were frontloaded instead. Yeah it sold way better than any other Capcom game ever but all that money was confined to one quarter in a time when games really need to continue selling well for several.
I think that's why Capcom is worried, anyway. They probably quite liked that on Nintendo consoles their Monster Hunter games would sell millions at launch and still sell hundreds of thousands every quarter.