Mainly because they’ve done better at it than the big tech forums have. Bottom line social media is in absurdly hard category for anyone to break into once someone else has a grip. Not sure if you remember google’s huge flop when they tried to step in facebooks terf with google plus (Which I do have to say, from a design and functionality level, was a decade ahead of facebook, but google’s attempt to shove it down everyone’s throats, was horrendous).

Bottom line, if you don’t care about who’s running it, the best social media platform, is the one that everyone’s already using. The existing company has to REALLY fsck it up big time to actually lose that advantage, and the competition has to, at least appear to be a trustworthy company. MS, Google, Facebook etc… would not have a good trust factor if they were to try to make a reddit like website, and reddit’s just not done enough to scare off the common users to leave a power vaccume that will draw an audience bigger than say what we have here on lemmy.