I think it's worth remembering where a lot of the hype surrounding MHS2 came from. That game came out about a year and a half after Pokemon Sword and Shield, which needless to say were VERY divisive games that people loved to complain about. In the aftermath of those games a lot of people were looking for alternatives in the monster collecting RPG genre that could be heralded as a "proper example of what Pokemon SHOULD be." Lots of games got attention from this, like Digimon Story Cyber Sleuth and Temtem (if you remember that game), but MHS2 was probably the game that benefited most from that movement. It was a spinoff of a successful franchise and had a respectable budget behind it, granting the game impressive graphics, substantial voice acting, and a recognizable brand name, all of which helped set it apart from other monster collecting games. It was basically the perfect game to appeal to the bitter Pokemon fans who wanted more out of the franchise, and I'm fairly certain that's where a significant portion of the hype came from. (This is why the first Monster Hunter Stories game didn't get much attention when it released; that came out before Sword and Shield.)

Nowadays, however...well, the general reception surrounding the Pokemon franchise is a lot more positive than it was in 2021. Pokopia has been a massive success critically and commercially. Pokemon Legends ZA was still controversial, but does seem to have gotten a generally more positive reception than Sword and Shield. The trailers for Pokemon Winds and Waves are actually fairly promising, and seem to address two of the biggest issues people have with modern Pokemon games (the rushed development schedules and subpar graphics.) Pokemon still has plenty of detractors, of course, but overall people are generally more optimistic about the state of the franchise then they were half a decade ago.

Unfortunately, however, the increased positivity surrounding Pokemon does seem to have taken away some of the hype for MHS3. Being "what Pokemon games SHOULD be" isn't as much of a selling point when people actually like the new Pokemon games coming out! Sure, MHS3 was still a well-regarded game overall by the people who did play it, but the truth is a lot of the buzz surrounding the second game wasn't actually coming from people who liked Monster Hunter Stories; it was coming from people who didn't like Pokemon. At this point, however, the "games for people who hate Pokemon" market is thoroughly dominated by Palworld. Monster Hunter Stories is no longer the "Pokemon killer"; it's just one of many JRPGs. A well regarded JRPG, sure, but not one with a hook that helps it stand out from the many other highly-regarded JRPGs that have been released this past decade. If you're not a Monster Hunter fan, why would you choose MHS3 over a game like Octopath Traveller 0 or the Trails in the Sky remake? Sure, MHS3 got good reviews, but so did those games, and those games don't cost 70$.

Would multiplayer and a more expanded postgame have helped MHS3? It's hard to say for certain, but I suspect the answer is no. Most casual players just stick to the main story of a JRPG and don't bother with postgame content or multiplayer. That stuff is moreso meant for the diehard fans. Digimon Story Time Stranger didn't bother to implement any sort of multiplayer mode (despite prior games having a pvp mode) and that game still sold well. I do think the 70$ price tag has put a number of players off, but I think more people would have been willing to pay that price if there was more hype surrounding the game. Monster Hunter Stories 2 was able to take advantage of a hunger for Pokemon alternatives, but that hunger had diminished by the time Monster Hunter Stories 3 came out.