I've read various explanations over the years. Too bad a p-38 ace in the Pacific wasn't transferred to Europe to fly p-38 and give us an apples-apples comparison.
The 109, 190, and 205 were better than the Zero. And the Japanese wasted their good pilots and were unable to replace them.
Wiki has a good explanation under the P-38 Operational History European Theater:
https://en.wikipedia.org/wiki/Lockheed_P-38_Lightning#Operational_history
One of the problems with the P-38 in Europe was that it suffered from an inferior heating-defrosting system. At high altitude during bomber escort missions, the canopy would fog/ice up and cause the pilot visual problems along with the distraction of trying to scrape it off.
Obviously, that was no problem in the Pacific.
They had more opportunities to perform in the other theaters
There were some biases that worked against the P-38 in England that played a role with its more limited use. The genesis of this might well lay with the first lot of P-38Es that were delivered to the R.A.F. in 1941. These first models did not have the counter rotating propellers or superchargers. This really took some of the fine handling characteristics away from the aircraft.
This British sentiment was bolstered by Major General James E Chaney who was commander on U. S. Army forces on the British Isles before the establishment of ETOUSA. He didn't like the thought of using the P-38 as a fighter either. When he found out that many of the fighter groups being assigned to England were to be fitted with the 38 and the P-39 he wrote Air Marshal Portal with his doubts of the use of the P-38 in England. He claimed that it was better suited as an interceptor to shoot down enemy bombers rather than an air superiority fighter.
Finally, there is a timing issue here as well. As Eaker was trying to equip his pursuit units he knew that the bulk of the P-38s being sent to Europe would end up going to the African/Med theater. He therefore requested that the P-47 be designated as his long range escort fighter (with drop tanks they could go a fair way but not quite as far as the P-38). So in a way, the P-38 was muscled out in England by a another very good aircraft. When the P-51 started showing up with the Merlin/RR engine, then there was even less room for the Lightning.
In doing a little bit of poking around concerning this question I ran across this dissertation by Bernard Boylan on the Air Force Historical Research Agency's website. I only skimmed it of course, but it looks like it is worth revisiting and reading in full.