A lot of people expect social media to serve them things to read, rather than following specific sites, and bloggers have a much keener sense of what will be rewarded by subscribers. In the old days, you could make a bit of money just from views, and there were many more places to make money from writing and speaking offline. There were also more long-form musings about academic life which today would be snarky posts on Bluesky. As posting on microblog sites became sometimes professionally useful, academics put their energy into that and let their longform blogs fade (or just got older and busier and were not replaced by younger academic bloggers).