One of the reasons for the decrease in activity is Hardfork 20 @coingecko.
After Hardfork 20, every account that had less than 20 Steem Power could barely comment.
Another thing is that Steemit Inc., stopped reviewing and approving new accounts toward the end of 2018, but if you check January 2019, there were a lot of new approvals by @steemit.
From what we know, @steemit is not the only way someone can get a new account on Steem. However, the information is hard to find even for regular users of Steem.