Twitter has hundreds of times more accounts. It depends on the audience, everyone's mileage may vary, so on and so forth, but as a general rule of thumb, if you're drawing from a pool that's 300x bigger, it's going to be easier to get orders of magnitude more followers. This is not Mastodon's fault; network effects are what they are.
Having said that, anecdotally I find it very hard to get people outside of a relatively narrow demographic to get on board with it. More than one fairly technically literate friend of mine has stopped about at the point where they're confronted with the choice of what instance to create an account on, especially if they choose and then get told "no, that instance is full, go create one somewhere else." It's not clear that the instance, at least theoretically, doesn't matter.
Note I say "theoretically," because it turns out it can matter. If the instance you choose is maintained poorly, your experience may be worse. (This is in fact my experience.) And it's not clear how to migrate between instances while keeping your social graph intact. Mastodon has a long way to go in terms of user experience to be accessible to people who aren't dedicated enough to the idea of a decentralized Twitter alternative to put up with this stuff.
Last but not least: it's hard for me not to notice that in the year or so I've had a Mastodon account, it's been very hard for me to discover people to follow, especially after the occasional burst of new users dies down. The people I followed are, except for two or three folks who are clearly dedicated to it over Twitter for philosophical reasons, mostly...just not using it now. (Of the two dozen people I followed, my timeline is now pretty much just content from two users, one of who is only "there" because his blog crossposts automatically.) You know where most of them are still active? Twitter.
tl;dr: it might be great for a project like OpenBench to also be on Mastodon, but not exclusively be on it.