Despite there being a lot of speculation, the truth is that we do not know why Tesla has not accepted new participants from the US in the FSD beta program since Dec 2021.
The Tesla CEO has stated an intention of welcoming additional participants on more than one occasion, but it has not happened.
Recently, Tesla added new FSD beta participants from our friends in Canada, fueling speculation that the current pause on adding US participants was based on there being a sufficient number of participants to provide suitable coverage for US roads. But that is merely speculation.