Yeah there are a lot of little usability things that give mastodon a lot more friction to use compared to twitter. Sure a tech savvy person can figure it out and breeze through it but for an average user being forced to choose an instance, having to figure out how to find people on other servers that arent coming in, and using other websites to search and browser addons is a lot of work.

The site has gotten a lot more usable recently, but during the first few twitter exoduses Mastodon was very rough and it's no wonder that average users bounced off of it.

Thats not even getting into the network effect. Sure some people are there for the microblogs from anyone and building out the network effect. But twitter became mainstream with professionals. So for a lot of people the appeal of twitter was getting direct contact from celebrities and famous people, getting news from reporters as it was happening,Insight from industry professionals like writers, artists, and directors, and more! You could even get better customer service from companies because their twitter reps were more empowered to help you since your issue was more public.

Then there were the niche subjects that being big leads to. Wrestling mastodon is like 70% one guy on my timeline who is just really enthusiastic about womens wrestling(and posts news and cards for men's stuff too probably because he notices how little is out there). On twitter you have lots of people tweeting about it along with the actual wrestlers themselves!