Centralized platforms offered solutions to both of those problems! And so they took over. That isn't to say they don't have other advantages, but those seem like the most significant reasons to me and my memory of how things went down.

Big reason #2 is moderation and spammers, especially bots. This is why "dead forums" couldn't just stick around as husks of their former selves; moderation is a sisyphean task against the relentless automatons, who would use any means of "interaction" to make the world a worse place.

Modest images are still significantly larger than big html, so the cost of webhosting became prohibitive for platforms without Investor Money. I think there was some vague idea that ISPs should provide hosting—and I believe some tried!—but webpage management didn't catch on with "the public"

Big reason #1 is image hosting. As internet speeds capable of handling large images and consumer digital cameras (incl. phones) became more popular, The Internet That Was completely failed to provide adequate storage. I think the idea that ISPs should handle that, but it never materialized.