Ah, yup, I stand corrected, they moved off of Apex. I told you not to quote me on that! ;)
Nevertheless, they still have capital requirements they have to adhere to in order to ensure settlement can occur, and it appears they were on the verge of being unable to meet those requirements.
https://blog.robinhood.com/news/2018/10/9/introducing-cleari...
https://www.cnbc.com/2021/01/28/robinhood-interactive-broker...