They did have restrictions on new users. People accused them of suppressing leftists by doing this.
> But Parler _didn't even try_ any of these less drastic measures.
Prior to the riot their failure was no greater than that of Facebook. Parler (but not Facebook) was forced offline almost immediately after it before being given any meaningful opportunity to do anything different in response.
It's also not obvious that their existing measures wouldn't have been as effective as alternatives once their user base stabilized and the backlog was cleared.
IIRC, Parler billed itself as an unmoderated "free speech" space [1]. IIRC, what little moderation it had was done by unpaid volunteer moderators, which in this case is akin to letting the inmates run the asylum.
[1] https://www.nytimes.com/2021/01/08/technology/parler-apple-g...: "Over the past several months, Parler has become one of the fastest-growing apps in the United States. Millions of President Trump’s supporters have flocked to it as Facebook and Twitter increasingly cracked down on posts that spread misinformation and incited violence, including muzzling Mr. Trump by removing his accounts this past week. By Saturday morning, Apple listed Parler as the No. 1 free app for its iPhones."
But Parler _didn't even try_ any of these less drastic measures. They could have properly moderated, or if that's not possible because they were growing so fast they could have limited posts from new users, or added moderation bots, or could have blanket-banned the most problematic subgroups. They didn't do these things.