An educated guess would be that the P-38 stopped operating at high altitude (it had a crappy cockpit heater), and in the Pacific, the Allison V12 work just fine at the altitudes that it was operating.
Got curious and did some searching- this seemed like a pretty good video. Evidently as the P-38’s engines increased in power, they reached a stage where they needed more propeller- which was much more complicated than one might think.