I am not an historian, but I went to Kinmen. The whole place is still, even up to this day, a fortress. There are anti-air bunkers at every roundabout, the whole seafront was cleared and covered with anti-barge traps and obstacles, roads were straightened to be used as landing strips for planes, and a LOT of military got stationed there. I rode my bike around, and saw so many soldiers, even though Taiwan and China are "technically" at peace now.

So I can imagine that part of it was just a brute display of force. The communists lost a lot of soldiers during the first and second battle of Kinmen, when the island was nowhere near as strongly defended, and they probably knew they wouldn't manage to get all the way to Taiwan anyway.

The main battle of Kinmen took place during the last stages of the military conflict of the Chinese civil war. Kinmen was-and is-a strategic location for any invasion of Taiwan. The Chinese Communists (CCP) wanted to end the war conclusively by conquering the last Chinese Nationalist (KMT) stronghold in Taiwan. Kinmen, by effectively acting as a guard post between the shortest sea distance from Taiwan and the mainland, needed to be taken out.

Both the CCP and the KMT realized this. The KMT proceeded to fortify the island as heavily as possible. They actually deployed one of their few tank battalions there, and received the aid of some former IJA advisors, including Nemoto Hiroshi, a former lieutenant general whom was experienced in amphibious assault. These advisers gave the KMT insight as to possible landing sites and how to handle the beachheads..

The CCP did not have a navy at this time. The one ship from the navy that had joined the CCP was a former British light cruiser that was sunk by KMT air attack. For the most part, all CCP amphibious action had to take place through junks and barges. These were hideously vulnerable to machine guns, let alone cannon.

The CCP also was looking to take the island through sheer momentum. The troops picked for the attack were essentially given only small arms. They couldn't even bring any heavy artillery pieces. These would be of little use against fortifications or tanks.

Needless to say, no matter how brave or experienced your troops are, you can't exactly expect them to fare well against heavily entrenched troops with armor and air support when they have only small arms without any naval or air support. Especially when you give them supplies for only one day. The entire CCP landing force was destroyed over the course of the battle, and since that battle the CCP's momentum was effectively halted. Then, the Korean war started, and the US signed a security treaty with the KMT, effectively stopping any further CCP attacks against Taiwan, and by extension, Kinmen.

Firstly, when the Communist Party declared itself victorious in 1949, it still hadn't fully taken control of the extreme south of the country. The battles for control of Hainan, Taiwan and the other islands including Kinmen were still being fought in 1950 - that is, when the Korean war broke out.

Up until that point, the US had been generally unwilling to throw much weight behind the KMT, in large part to avoid being drawn into a conflict that could escalate into a war with the USSR. North Korea's invasion of the South was seen to demonstrate that this approach was simply untenable, and that Communism would be spread through force if not stopped.

At that point, Truman - who was now obviously under huge pressure to intervene - ordered the US Navy to blockade the Taiwan strait and prevent any further military action by either side, which it has continued to do to this day.

tl;dr: The US intervened halfway through the fight for the southern islands, making it clear to the Communists that any further expansion would mean war.