I think there's a few reasons for Sunbreak's launch being as soft as it has:
1. Lack of content in Rise: Rise didn't engage people for as long as Monster Hunter World did. MHW was designed as a "lite" GaaS title, with daily login bonuses, procedurally-generated quests, and constant little updates to keep people coming back for post-game stuff. The game also did a better job of curating its content, and making every new monster feel like an event. In contrast, Rise was released with a bunch of its end-game content missing, provided very little in the form of a post-game grind, and took a rather stripped-down, oldschool approach to how it presented all of its content. As a result, engagement dropped off quicker, and it's always more difficult to get people to come back to a game they've stopped playing for good.
2. A lack of impressive new features in Sunbreak: Capcom has always called Sunbreak a "massive expansion," which is the same term they used for Iceborne. However, in contrast to Iceborne, Sunbreak doesn't actually feel like a massive expansion. There are a bunch of returning monsters and a handful of new ones, but two core issues still remain: A) The lack of a proper post-game grind, and B) The lack of anything that really makes you go, "Wow, that's a super cool new thing." For better or worse, Sunbreak is just more Rise and with less content than IB.
Point no. 2 also ties into marketing, which is where its effects were most felt. I don't think Sunbreak has been marketed particularly well. Save for that final trailer during the Mini Direct, the other showings have felt rather underwhelming, and they definitely tempered people's expectations for Sunbreak. In contrast to SB, Iceborne had multiple showings, a couple of lengthy streams, and a lot more excitement around its launch.
TL;DR: I think people were willing to forgive MH Rise as a relatively content-light experience (or were simply being exposed to it for the first time as new fans), but I don't think they were willing to do the same thing a second time around with Sunbreak. I love the game dearly, but I do personally feel Capcom dropped the ball on Sunbreak rather hard.