First and foremost, I disagree that the USMC fought "most short and limited periods of conflict." As a US Army officer myself, I am obligated to dispel the myth that "the Marines take ground and the Army holds it," or other such similar nonsense that gets trotted around online. The Marine Corps and the Army are very different organizations that are equipped, trained, and structured differently to fill different roles. Throughout American history, the US Army has been the nation's predominant ground combat force, even for minor conflicts. The USMC had extremely limited involvement in the Indian Wars, for example, which accounted for over a century of nearly constant, low-level conflict by the US Army. In the 20th century, there were other minor conflicts in which the USMC played a much smaller role; for example, during the invasions of Grenada and Panama in 1983 and 1989, respectively, the ground forces were almost entirely Army, not Marines. This is due primarily to two factors: relative size, and location. Compare the relative sizes of the Army and Marine Corps throughout American history (note that these numbers are for active duty personnel; the modern US Army can double or more in size when including Reserve and Guard personnel, the other services have much smaller reserve components). The Army has consistently been an order of magnitude larger that the Marines, so naturally the Army did most of the fighting. As far as location, until the start of the Cold War, Army personnel were predominantly located in various posts throughout the continental US, with the Philippines serving as a notable exception. Marines, on the other hand, are generally stationed in conjunction with the Navy. They're usually either on ships, or in coastal bases. This precluded them from participating in most fighting inside the continental US because they weren't located where the wars occurred.

However, you are correct that during the so-called "Banana Wars" in the early 20th century, the USMC fought several small wars largely without assistance from the US Army. So, why is this? It goes back to size and location. Those conflicts were mostly in small, coastal nations. Getting to these locations required going by sea, which the Army was not set up to easily do. It was possible, but not simple; just getting the Army from Florida to Cuba during the Spanish American War was a logistical nightmare. But Marines are already on ships. And to install a new regime in a country like the Dominican Republic, a force as large as the Army wasn't necessary. A small force of Marines, supported by the Navy, was all that was required. So why go through the trouble of mobilizing the Army, which is a cumbersome process, when the Marines were already present and able to accomplish the mission?

When and why did this change? The Cold War. With US bases all across the globe, the organic sea transport of the USMC gives them less of a monopoly on being in the right place at the right time. Before, if trouble arose at some remote US interest, the closest US military force was likely a Navy ship and its Marine detachment. Now, however, there's Army personnel scattered across the world with access to airplanes and helicopters. It's easier for everyone to get places, which opens up more options for policy-makers.

I think there are several things to unpack here. Based on the wording of the question, it sounds like you are saying that the US generally used the Marines, while now the Army is the one called into action, instead? Perhaps I'm misreading that.

To break down the answer a bit, the Marine Corps has still been the primary "ready" force for rapid deployment on large scale anywhere in the world. There are several reasons for this: they have the manpower capability (to include aviation, formerly tanks, and other vehicles); they are part of the Navy, which means they can be deployed nearly anywhere near a coastline, and since the US Navy has a presence virtually everywhere on the globe, the Marines being aboard the ships makes it a natural fit to be ready to deploy them.

To your question of "fairly recently," I'd have to say that the Corps is still primarily used in the way they have been for centuries. From Beirut, Grenada, Somalia, etc. but I think the root of your question has to do with both WWII and the Cold War.

In WWI, Marines were attached to the 2nd Infantry Division, and were not sent over in their entirety. Between the wars they were used considerably in various interventions in the Caribbean and Central America. WWII, however, dramatically altered the use of the Marines. The Pacific Theater was the proving ground, so-to-speak. There was talk among the branches of the military to possibly do away with the Marine Corps entirely, or at minimum wondering if the old way (of landing men in boats on a beachhead) would work in the more modern age. The Marines holding of Guadalcanal after being cut off by the Japanese at sea, as well as their successful, albeit bloody, takeover of places like Tarawa and the Solomons, not to mention their valiant stand at Wake Island and Bataan, proved that the Marines still had it. The Pacific Theater was well suited to what they did best. But, the Marine Corps was still dwarfed by the Army even in the Pacific. Twenty-two US Army Divisions were sent to the Pacific, where only 6 Marine Divisions were. Both still operated by their basic means, which proved in some cases, like the Marianas, to be detrimental to the other. The Marines' tactics are speed combined with ferocity. Hit the enemy hard and fast. The Army's is generally more build up, press forward, find gaps or weaknesses and pursue them, take more time rather than go in quickly. During the Marianas campaign this was seen all too well, as the Marines were quickly gaining ground on Saipan while the Army took a much long time than the Marine leadership thought it should on Guam.

By 1948, when the Soviet threat in Berlin and elsewhere seemed more imminent, the US began to conscript again in peacetime, which had always been controversial. Through the Cold War, it was felt necessary to maintain a higher level of "readiness," however, the Marine Corps, as a smaller branch, has always been able to maintain this "readiness" without needing much help. When the Korean War broke out, US trops stationed in Japan, like the 1st Cav Division, were deployed because they were all we really had initially. The invasion of Inchon in September, 1950, barely 3 months after the fighting began, Marines were sent on an amphibious invasion to secure the beachhead, which they did. As is typically the case, the Marines were not totally by themselves. They were supported by ROK Marines and US Army soldiers. The same was true in many Pacific Battles in WWII: the Marines secured the beachhead, and held, then the Army took over and continued the campaign.

There's nothing unusual then, about Marines being first landed at Da Nang in 1965, followed not long after by US Army divisions. Since the early '60s, the Marine Corps has maintained a relatively stable number of personnel (around 200,000 not counting the major build-up during Vietnam). There is certainly an element of pride and ego that goes into this as well. Marines are trained to think of themselves as different. "Every Marine a rifleman," "The Few, The Proud," "there is no such thing as an ex-Marine, only a former Marine," etc. These things have fuel for recruitment for more than one generation.

So, perhaps more to the point of the original question (sorry, I feel like I've rambled in this one more than usual-its been a minute since I answered a question), The Marines are still the main ready force we have, precisely because 1) they've been doing this a long time and have the data to prove they know what they are doing 2) they are a smaller, well-equipped (depending on who you ask) force designed for combat action almost anywhere in the world on a relatively short time frame. If the Marines had to maintain levels like the full US Army AND fully mobilize and engage in the same way as the Army, they'd face the same logistical difficulties. As is, they are larger than a single Ranger Battalion, much larger than a team of SEALs, but much smaller than the entire Army. So it works that they are the "go-to" still for medium scale actions, or ones that are needed fast.