If I had to guess?

1) No porn... I know, but let's be honest, half the reason people join sites like Reddit in the first place is their genitals, then the rest is just gravy.
2) Younger people who want to join forums like this tend to be Left Leaning American style. This place is where people who upset the American Left end up. Ergo - they don't want to come here.
3) If you're not after porn and you're not looking for an echo chamber for your thoughts, and you want to join a place for conversation, you have a choice of Reddit, which everyone knows about and has millions of users, or Saidit, with enough users that I know most of their usernames.
4) If you search Google, the most popular search engine, for a place to chat with other people, you'll get Reddit, Facebook, Twitter, Snapchat... By choice or by popularity, Saidit won't come up straight away, so unless you're specifically looking to avoid those other locations, you won't search long enough to find Saidit.
5) It doesn't offer much to the average person beyond what Reddit does. Sure, you and I know it offers a chance to speak without being cancelled or banned, but to a "Normie", that doesn't mean much. Why sign up for Saidit over Reddit?
6) Maybe most importantly, the kind of person who spends a lot of time on these kind of sites often does so for validation. They want precious upvotes to make them feel good. They want public applause and acclaim, to see their fellow man tell them "You're right and what you said was good". People on Saidit don't tend to be the kind of people who care about that kind of interaction. As a result, no motivation to join and post... After all, if total strangers aren't clicking the arrow next to your name, why live?

Those would be my first guesses.