It's no surprise to me that this Stack Exchange website was not chosen to graduated from beta, even though the requirements to do so are very low. This website has been falling into dereliction over the past several years.
Some ideas on why this website is failing:
Carey Gregory's answer is blaming COVID for causing the loss of a number of high-rep users in his answer, but the retention of experts has always been an issue, and I'd argue that COVID should instead have help revive this website due to the high interest of the public in the many COVID-related medical questions.