Lemmy, reddit, and other forum type websites are primarily text based. Microblogging and social media are short form and have plenty of pictures and videos, which catches and retains the attention of users more. Their algorithms are also more intense and curated compared to say reddit’s front page which is more generalized to all its users. Forums mean you have to spend time reading and typing while you can doomscroll and press a reaction button on social media and microblogging.