Some people think Mastodon is too weird to become popular:
I joined Twitter in 2008. We had to put a "d" in front of a tweet to convert it to a direct message and every other day you had some embarrassing private moments exposed because someone forgot about the “d”. Hashtags were just a community hack, introduced by Chris Messina to somehow tag content. Search? Hah, you wish! Tweets via SMS were supposed to be a thing. Oh, and the daily meet and greet with the failwhale. Totally not weird.
@bastianallgeier everything is weird and unpopular until it gets accessible enough for normies.
Than it becomes the new mainstream and everyone gets surprised as if we haven't seen this happen since the dawn of time.
Mastodon is free, easy to use. Considered cooler then it's counterpart. Normies can feel like part of the "new big thing" by joining. It's not a big brain idea to understand why Mastodon is growing.
Also, check this to see the Masto's growth in real time. @mastodonusercount