Why is Jiang Jieshi generally seen in a positive light?

This is an interesting question, but I think that you are conflating two different things here: Jiang Jieshi (Or Chiang Kai-shek as he is often called in the West) being seen in a positive light, and Jiang Jieshi not being mentioned or overtly criticized in mainstream history. These are two very different things. To the first:

Chiang Kai-shek was never and since his time ever been really portrayed in a positive light. His ideology, called the, "New Life Movement" encompassed ideas from Sun Yat-sen, foreign missionaries, and Confucianism. His Nationalist party was formed along Leninist lines, and was incredibly ruthless in employing the Blue Shirts, who morphed into a military apparatus and secret police that was used to investigate all sorts of people and movements thought to be subversive. At the same time, Chiang Kai-shek was extolling the virtues of leaders in fascist Germany and Italy, as well as Stalin in Russia. So, what was his saving grace? There were a few. Most notable, though, was his marriage to Meiling Soong, a Wellsely College graduate from Boston from a very rich, very Christian family that helped to get recognition from the United States Government, legitimizing his government at a time when he had not had full control. Next, he presided over the "Nanking Republic", from 1928 - 1937 which on the outside appeared to be China finally rising up (Most of these economic ideas that led to this, "Golden Decade" came from his Harvard educated brother in law). At this point in time he was thought of as the man who would finally bring China into modern times. Most importantly, though, was the entrance of the United States of America into World War 2, which immediately elevated Chiang Kai-Shek to the president of a "great power". This was quickly followed by military advice, massive loans, and equipment and aviation fuel flown over the mountains of India. Technically at the time there were three governments that existed in China, one under the Communists operating out of Yan'an, one out of Chongqing under Chiang Kai-shek, and another operated by Chinese who were cooperating with the Japanese. Only Chiang Kai-shek's government was seen as legitimate, so all aid and equipment went directly to him. However, even at this point in time he was not viewed in a positive light. One of the biggest points of contention between the American general Joseph "Vinegar Joe" Stilwell was Chiang's reluctance to commit Nationalist troops to the fight with the Japanese, since he was expecting an Allied victory and wanted to put down the Communists as quickly as possible after the Japanese were dealt with. A quote from Orville Schell's Wealth and Power: China's Long March to the 21st Century:

Stilwell referred derogatorily to Chiang as "the Peanut" and openly chastised him as "a stubborn, prejudiced, conceited, despot" surrounded by "yes-men."

It was also at this time that that Chiang published China's Destiny which John S. Service called Chiang's Mein Kampf. Furthermore, the American Journalists Theodore White and Edgar Snow both were reporting negative things about Chiang Kai-shek and his Nationalist government.
After the war, the Civil war continued, and it is true at this point that the United States public were sympathetic with Chiang, but at the time he was governing a fragmented country that quickly slipped out of his control. It should be noted here, though, that the United States of America did attempt to facilitate a meeting between Mao and Chiang, carried out by U.S. Ambassador Patrick Hurley. It failed, however, and by 1949 Chiang had been abandoned by the majority of intellectuals, students, professional classes, and urban workers. He fled to Taiwan later in the same year. At this point he was labeled as, "The man who lost China". Another quote from Orville Schell's book:

When Chiang finally fled to Taiwan in 1939 to be labeled "the man who lost China,' leading historians on both sides increasingly began to view the whole republican era as just one more unsuccessful experiment in China's series of efforts to reinvent itself.

As you can see from above, the view about Chiang is not very positive (though some have made the point that he did the best he could with the hand he was dealt. See: Jonathan Fenby's Chiang Kai Shek: China's Generalissimo and the Nation He Lost).

Concerning your second question, though, why was he not put into the same class as leaders such as Mao, Stalin, or Pol pot? That has more to do with his own irrelevancy than anything. Once he left China and took up post in Taiwan, he essentially went to the man sitting next to Churchill and Roosevelt at the Cairo Conference (seen in this picture here ) to, as Mr. Schell puts it:

One left his[Chiang's] palace with a sense of having been in the presence of a tragic figure, a proud man left to cope with the indignities of having fallen so far. Like so many other Chinese leaders, Chiang had spent his life seeking to restore his people to greatness by appealing to them to "save our country and make China strong and independent, with a position of equality among the other nations of the world," as he put it. But here he was, marooned on an offshore island, a living example of yet another failed attempt to restore wealth and power to China.

In fact, at this point the narrative around Taiwan no longer centered around Chiang, but simply against allowing Communist expansion. In fact, if I recall correctly, Taiwan was not even formally notified about the Shanghai Communique by the United States, but instead by Zhou Enlai, Premier of the PRC.

I apologize if this is long and confusing to read. My two main points that I was attempting to make were, 1. Chiang Kai-shek, while at one time viewed favorably, does not and did not really maintain any positive reputation and 2. Most people do not speak too much about Chiang Kai-shek's crimes against humanity because he quickly became politically irrelevant, and at one time was one of the United States' fellow "great powers" president.