Because the Pacific was largely a naval war and aircrafts needed to be launchable from carriers. This meant specific requirements like being able to take-off from short runways and designed to be transportable aboard aircraft carriers were needed for airplanes in the Pacific. A P-51 for instance would not be able to takeoff from a carrier and thus not suitable for a naval war. The Hellcat on the other hand would be able to. This trend continues today in which the navy operates F-18s and F-14s which could be operated from carriers whereas the USAF F-15 isn't designed for it.

It is something of an oversimplification to say that the planes used in the two theaters of WWII were different.

Most, if not all, of the US planes that saw service in Europe were also used in the Pacific. B-17s flew missions against Japanese carriers in 1942. Mustangs escorted B-29s in 1945. So on and so forth.

It's not wrong, however, to say that the US Navy and the Army Air Forces had different types of planes in general. Simply, that's because the two services had drastically different needs.

The Navy, knowing they'd be primarily operating from carriers, needed sturdy aircraft that could take the beating a carrier landing would inflict. They also tended towards the more durable, but usually less powerful, radial engine... stories abound of planes like the Corsair or Hellcat having entire cylinders shot out and still keep flying.

Meanwhile, the USAAF favored the more powerful but on the whole more fragile inline engine. This also allowed for better aerodynamics, since your average inline engine was smaller than a radial... compare the Mustang and the Thunderbolt, for example. Better aerodynamics and more power meant faster airplanes, which was important in the skies over Europe as the Germans tended towards speed and less towards maneuverability (again, in general).

In the Pacific, Japan tended to have highly nimble but less fast planes. While no smart pilot would willingly dogfight a Zero, the Hellcat or Corsair could at least attempt it without being shot down immediately. They were better off using their other strengths, however.

So... since the Pacific was, basically, a naval war, it tends to reason that the US Navy's planes would be featured there, even though the USAAF played a major role as well.

Likewise, since Europe was mostly land combat, the USAAF's aircraft took pride of place. The US Navy planes did have a role to play as well, in attacks in France with the UK's Royal Navy.

On a similar note, I believe that when Britain offered the USSR a supply of Spitfires, the Russian pilots tried them out and weren't all that impressed, whereas they were very fond of the P-39 Airacobra which had been trialled and rejected by the RAF.

Whether this was due to the differing demands of combat operations in the two theatres, or just a difference in preferred tactics and doctrines, someone better qualified than me may be able to say.