Even if the domino theory was marginal and only Viet Nam fell it was enough. The Soviets wanted a warm water port in the Pacific from which to launch intelligence vessels and warships. The nuclear and space races were hot and getting hotter and they wanted to be able to not only monitor any and all of our progress in Micronesia where we had bases but to get a foothold in the area. The same way they were in Cuba and other areas around the globe where they could choke off major sea lanes or monitor our progress.
A point almost everyone ignores.
In later years the purpose of Nicaragua was that it was the next logical place for another canal across Latin America. Another choke point and bases from which they could send recon and bomber fights up our west coast which was too far from Cuba.
In 1984, when Reagan was running for re-election, he could and did boast that "not one inch of territory" had been lost to the Communists on his watch. It was not just a straw man. It was a real turnaround from the prevailing trend.