I always imagined it was something to do with the laminar flow wing (and high power to weight ratio of the P-51 particularly) inducing these stall/spins.
My thinking being boundary layer separation and therefore low speed instability occur much more readily and suddenly in aircraft with them. Compare to something like a Stuka or hurricane with a thick leading edge which can dance around all day, but not particularly fast nor frugal. Engineering is always a compromise.
The P-51 was designed for speed, e-retention and most importantly endurance, to babysit the bombers to Berlin and back.
Please correct me if I'm wrong, merely a layman here.
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