Money talks.
I don't think they care enough about the show compared to getting signed to Disney/Marvel and being on Broadway or compared to being low-balled by Netflix and having to endure the Heartstopper fan base any longer as they grow up and move on.
Plus this is Netflix, how many hugely successful shows get capped at the 3rd season?
Heartstoppers second season didn't perform as well as the first and the 3rd probably won't do as well as that, law of diminishing returns.
A combination of being out of the pandemic, the fans of the first season growing up in the space between the 2 seasons (hard to believe but I'm sure some must have!), and a total lack of fan engagement with the cast since the majority of them had been chased off social media since the first one, probably all played a part.
I think it could get a 4th season at a push, but we'll have a long wait for it and that would be the end of it for sure.