The Marines are a unified, organic command (not unlike the IDF) that can put an all-arms force on the ground anywhere faster than everyone else. They are an exceedingly flexible force and the largest elite force in the world.
There are about three other armies in the world that maintain the kind of high-volume, high-quality armored forces that the Marines probably ought not to tangle with and leave instead for the U.S. Army. The other 200 countries out there have more to fear from a Marine Expeditionary Unit showing up in three weeks or less than from an armored corps that takes 100 days (and larger air and sea fleets) to travel and assemble itself within striking distance of its objective.
That is why Marines are rising to the top, because the past twenty five years of combat have rewarded the highly trained, the most mobile, and the ones who can put the most firepower at the right place, first. The Army's toys are too heavy and expensive to do that, and they are already relying heavily on their handful of elite and specially trained units to do Marine-like jobs, because there are no jobs for tank divisions and heavy artillery units.