A great question and I am not sure I have seen a satisfactory answer. Many XP aircraft like the XP-60, 62, 72, XF7F, XF8B, and XF2B, all had spinners. In addition in Germany, the Fw-190, and Ta-152 (although this one had a liquid cooled engine) had spinners and the British Sea Fury also. Also, a few Japanese like the Zero, had spinners.
In general, in the US, with its powerful radials, decided the extra cleanness was off set by complexity, I guess.