there's certainly a higher rejection rate for github PRs

That seems unsurprising given that it’s the easiest way for most people to do it. Almost any kind of obstacle will filter out the bottom X% of low effort sludge.

correlation, not causation.

Lowest common denominator way will always get worst quality

sure it's correlation, but the signal-to-noise ratio is low enough that if you send it in via github PR, there's a solid chance of it being ignored for months / years before someone decides to take a look.