People are lazy. Getting people to sign up for a forum has a much MUCH higher inertia than just clicking join in a group on a platform they already have an account for.

People will subsequently evidently just "deal" with it's inadequacies.

Reddit has the same advantage, you have one account and subsequently have access to a billion and then some communities. Ditto for Discord versus self hosted solutions like Teamspeak.

Lemmy kind of adresses all of this, but actual forum software I think is still mostly the same as it was in the early to mid 2000's when I used it. It's demise is a shame but not a surprise.