In a nutshell, he simply didn't have the military capabilities required.

He wanted to. He was very serious about it. But he needed the US to help and the US didn't want to. Chiang had big plans, but they were unrealistic plans that called for logistics support from the US.

In Chiang's radio broadcast speech to the people on May 10th, 1949, he sworn, "one year to prepare, two years to attack, three years to sweep, 5 years to success."

However, the then president of the United States, Harry S. Truman, has already decided to give up on Chiang. From the book "Off the Record: The Private Papers of Harry S. Truman", we know that Truman was never a big fan of Chiang. He went as far as to state, with regards to the civil war between the communists and the nationalists, that "we are not mixing in China's internal affairs."

The Korean War forced Truman to take an 180 on his policy, but there were clear bounds to what the US was willing to do in support of Chiang. The US knew Chiang was intent on retaking the mainland.

In a statement issued on June 27, 1950, Truman said:

Accordingly I have ordered the Seventh Fleet to prevent any attack on Formosa. As a corollary of this action I am calling upon the Chinese Government on Formosa to cease all air and sea operations against the mainland. The Seventh Fleet will see that this is done. The determination of the future status of Formosa must await the restoration of security in the Pacific, a peace settlement with Japan, or consideration by the United Nations.

It was clear to Chiang that if he was to retake the mainland, he was on his own.

The US position was made even clearer when the US signed mutual defense treaties with Japan, South Korea, Philippines, Australia and New Zealand, but not with Chiang (Republic of China). This was unsettling to Chiang, so between 1953 and 1954, Chiang sent his best diplomat to talk with the US ambassador about a mutual defense treaty.

Eventually, when the treaty was signed, it was made clear that the US will only help defend Taiwan and the Pescadores (i.e. not even Kinmen and Matsu) and that the US will not help Chiang if he was to initiate military action against mainland China.

Chiang considered this a great diplomatic victory, and wrote in his diary that this establishes Taiwan as a secure base of operations for retaking the mainland, a God-given opportunity.

However, over the next decade, it was more and more apparent that the US will not only refuse to aid in any military action against the mainland, but also actively monitor and influence Chiang's ability to take such action. This is because the US has pivoted towards a much more cautious doctrine that emphsizes on preventing a nuclear war.

During the 1960s, Chiang even drew up actual invasion plans and had his son bring them to Washington and sell them to President Kennedy. After the Kennedy assassination (not saying Chiang did it) Chiang kept pestering US officials and kept getting a boot in his face due to his unrealistic war plans.

The US knew that most of Chiang's covert scouting parties have resulted in high casulties and that none of Chiang's plans had any hope of success.

It was during this time and under these circumstances, that Chiang established the Project National Glory. I personally knew a then company commander who was recruited for the project, but turned it down because he was getting married. He revealed that they (the officers) were promised cash rewards paid in advance to their families. Anecdote has it that in one amphibious landing exercise held under bad weather because Chiang wanted to showcase his elite soldiers to US officers, Chiang ordered the exercise to continue even after one landing craft capsized. It wasn't until the US officer used some very strong words and threatened to walk away from the observation of the exercise that Chiang called off the remainder of the exercise. Several elite soldiers died that day.

By the 1970s, Chiang was too old to make it a reality. Before he died, he ordered that his casket to not be buried until the mainland is retaken. To this day, his casket is still unburried, and the place where the casket is kept is not called his tomb, but his "resting place".

By that time, the US was starting to warm up to China, the prospect of opening up the vast Chinese market was top priority of every western nation, and Taiwan itself was at a stage of economic transformation. Chiang's death settled the matter of whether to retake the mainland; his son (who succeeded as president after Chiang's death) realized full well that the ROC was stuck on Taiwan and the communists weren't going to collapse any time soon. So the possibility of ever retaking any part of the mainland basically died with old Chiang.

And that's why the nationalists still only hold Taiwan and the surrounding islands to this day.