For the people in private practice managing to fill with high rates, there's probably also some sort of specific draw that they've cultivated. They could be extremely good with patients while maintaining high standards of care (which means saying "no" in many cases.) They could have made a name locally/regionally as an expert in a particular diagnosis or diagnostic category (couple of frequent forum contributors fall into this category.) They could be willing to see more severely ill/risky patients than the typical outpatient PP doc. They could diagnose everyone with whatever they want to be diagnosed with and fall just barely on the line of not technically being a pill mill.
I know that it's important to me to maintain very high standards of care and to try and push back some on the diagnostic bloat trend, which means (often figuratively more than literally) saying "no" to patients sometimes. When you combine that with the the idea that I'm probably average with worried well patients, when compared to other psychiatrists, but not extremely good, it means I would not have as good a chance with the easiest side of high-earning PP (in terms of workload.) I'm not naturally effusive/highly emotive, which many of the worried well patients seem to want, but I'm best with patients who have serious problems and who need/want a serious, invested, respectful psychiatrist. So, if I ever wanted to aim for a higher-earning PP, I'd probably need to target specialized or severely ill populations. Meanwhile, I think that makes me a good fit for an employed generalist job, where my primary incentives align toward providing the best quality care possible and only secondarily align with patient satisfaction.