I am curious if hydroplaning velocity is a consideration? Approx = 9*SQRT(PSI)

That could definitely be it. It puts hydroplaning speed above instead of below touchdown speed for that class of plane.

I carry a portable battery pump that will handle both mains and go from 30-45 PSI as I see fit for the day. I'm pretty much never seeing rain. At 30, hydroplaning speed is below touchdown speed but at 45 it's near or above it.

Dang, I reckon I'm the only one that has calibrated eyeballs... see a little too much pooch, add air. Now what to do about my belly pooch

From what I remember, ply rating, gross weight, and location (MLG/NLG, TW) on a specific aircraft help to determine the pressure the aircraft manufacturer requires. Heck, I bet the oleo strut gear vs spring comes into play as well.

While at Sun-n-Fun ask tire manufacturers what the difference is between 4 ply and 6 ply tires. Most will tell you the only difference is the labeling on the side wall. They have to "offer" 4 ply tires because some TCDS specify a 4 ply tire. But the tire makers don't really make a 4 ply tire, only 6 ply.

George