Seems most likely to me that it's the onboarding process. I'm no technical nitwit, but onboarding the mastodon and Lemmy universe was frustrating and unintuitive for me. Your mileage may vary, but why not just get Registered at lemmy.nooby or lemmy.green at first and then shop for an instance you like when you finally get wtf instances are.
The barrier of entry is lower. You have to remember the average internet user is either not tech savvy enough, or too lazy to learn or step out of their comfort zone.
Then average person who manages to pass the first hurdle of joining mastodon sees that it's full of scary nerds instead of anyone they care about and never return...
The second image includes a reply from a semi-private account, so while I found the link it won't let you view it directly without a Bsky account. For those interested anyway, here's the link: https://bsky.app/profile/proggrrl.bsky.social/post/3lbrdyowbs22m