Because for the previous 15ish years before Musky got a hold of it it was actually decently regarded. Many people turned to it for up to date news and trusted it. It was a place where brands could interact with customers, celebrities with fans, etc. It had built up trust. People are having a hard time letting that go.
Even today there are top posts on Lemmy that are screen caps of Twitter. Not even Lemmy users can stop reading Twitter and upvoting. I down vote them on principle but it's 100 to 1.
Reddit and by extension Lemmy were designed to enable content aggregation. I dislike people having so few principles in general but the system is working as intended by users posting things they find interesting happening elsewhere.