Why were Marines chosen for Inchon?

I think a big part of it was due to the state of the Post-WW2 Army and who was actually prepared and immediatley available to conduct an Amphibious assault and what else was going on in Korea at the time the landings were conducted. The post WW2 Army was severly gutted and readiness/training of remaining units was not in good repair (see Task Force Smith). Most available Army units in the Far East had already been committed to the Pusan Perimeter and were barely hanging on. If you study the operation, it is interesting that the Marines were not MacArthur's first choice of troops. The original plan was to use 1st Cav Div, the unit that MacArthur relied heavily upon during his landings in WW2, but they were rerouted to holdling the Perimeter. The same happened to 2ID and 7ID (which was in reserve status and would need significant rebuilding) would not be ready in time. Stateside Army units could not be mobilized, trained, equipped and moved fast enough to conduct the operation as the hold on Pusan was tenous and the forces fighting there needed to be relieved ASAP. So who was left that was not already engaged and could realistically pull off this operation on relatively short notice with a good chance of success? The Marines were. So I would hesitate to read into MacArthur's choice of using Marines at Inchon to infer that "it was a job only Marines could do." It is more accurate that MacArthur, begrudgingly because there was still a lot of interservice rivarly happening, used the Marines because that's all he had left available who had the training to conduct Operation Chromite within the time frame needed. This is not taking anything away from the Corps. I've visited the Inchon landing sites on a staff ride. The Marines did a brilliant job of overcoming not only the North Koreans, but the unique geographic challenges of Inchon.

Because Doug was PISSED!

David Habersham's book "The Coldest Winter; America and the Korean War" goes into this very well. He postulates it was a political move on McArthur's part to bring in support from the Marines, especially related to the Joint Chiefs.
Very complicated relationships and I would have to go back and re-read that section, but if I remember correctly, by bringing in a Marine Division, McArthur could effectively gain support for the mission from another branch (and therefore the Marine Commandant).
I highly recommend this book BTW.