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I think the main reason is really an organisational one. USMC units were organised along the lines of combined arms battle groups where as the US Army units were designed to operate within a larger Corps framework.
The nature of the Pacific island hopping mean that each engagement needed a combined arms task group rather than an entire corps/army/army group like in the ETO. The smaller self contained units of the USMC were well suited to these operations.
As the Pacific Campaign operations grew in size more and more US Army units were involved such as Leyte and Okinawa. In many cases the Army units formed the follow on reserve to exploit the UMSC lodgement and establishment of the beachhead, as in Siapan the landed 2 days after the USMC, Peleliu about a week and on Guam they landed on the same day, in the afternoon but as they were not equipped as well as the Marines for amphibious assault they did have some difficulties ties before gaining a foothold, perhaps reinforcing the notion that the specialists should go first.
Once ashore of course the Army and Marine units generally advanced side by side, as on Guam where a complex pivot was carried out to roll up the island from South to North.
As much as we might like to think it was another conspiracy theory or turf war I think you'll find that, as in all things, it was simply a logical employment of a specialist force.
Why no Amtracs? Initially the Amtrac was not conceived as an assault craft, it was for ship to shore supplies, so that heavy stuff could get across the beach and into the hinterland.
Amtracs were very slow (6knts), very small (24 troops max) and very un-armoured. They were great for being able to drive over a reef and continue to the beach as they did at Tarawa but they were then shot to bits as they crawled along and had no armour. The Higgins boat were much larger and could carry a full platoon, could make 12knts and were armoured so were far better when there was no reef.
Later the Amtrac did get amour and were used in a number of operations in the ETO, Scheldt, Rhine and in Italy.
If one studies the ETO, most of the operations were combined arms operations involving leg infantry, mechanized infantry, armor and air support. Combined arms warfare was not the forte of the Marine Corps which was basically an amphibious force, and, I add, the best amphibious force in the world. They were primarily infantry, however.
