Twitter is easier for new users to join. If you aren’t on Twitter and want to be (or have left and want to give it another try), the flow to create a new account is really easy - go to twitter.com, and then the sign-up flow is pretty standard, something almost everyone nowadays is familiar with.
Want to join Mastodon? First you have to choose an instance - there are so many to choose from. If you are joining Mastodon because a lot of people you know/follow already have, it makes the choice easier-just join whichever one they are on-but then what happens if it is closed to new registrations? Whereas, if you just heard about this Mastodon thing and want to check it out, a lot of people (especially your average non-technical person) presented with “choose an instance” are going to respond “er, this thing looks too complicated, I can’t be bothered”
Want to join Mastodon? First you have to choose an instance - there are so many to choose from. If you are joining Mastodon because a lot of people you know/follow already have, it makes the choice easier-just join whichever one they are on-but then what happens if it is closed to new registrations? Whereas, if you just heard about this Mastodon thing and want to check it out, a lot of people (especially your average non-technical person) presented with “choose an instance” are going to respond “er, this thing looks too complicated, I can’t be bothered”