There are a variety of reasons, some being:
-Spain is an utterly irrelevant, poor, and war-ravaged country with no resources nor industry. There's no external pressure on them.
-Franco got rid of most of his potential rivals early in the war, the deaths of both Emilio Mola and José Sanjurjo in plane crashes in the early stages of the war can't be called an accident.
-He was very popular among his men, being a great colonel or subaltern, also gained a lot of prestige by relieving the siege of rebel troops in the Castle of Toledo.
-Franco did not mess with the exact policies, he was more of an arbiter figure, relegating the matters of government to ministers that he could replace whenever something didn't work.
-Franco's anticommunism made him approach the western camp during the Cold War. His positioning with the western block translated into a collective "hey, at least he's not a communist".
-The regime dropped most totalitarian and fascist aspects early, by the 50s fascist ministers were replaced with conservative technocrats, Spain stopped being a fascist-run country in the 50s, to become a military/conservative dictatorship.
-Franco was always a very careful man when it came to diplomacy, constantly replacing his foreign ministers according to the tide of WW2, switching from germanophiles to anglophiles, always trying to be with the winning side. This allowed Spain to survive WW2 by being close to the Axis but with no intent of ever joining the war against the west, albeit he could have joined only against the USSR in different circumpstances. Also, Spain was supporting the Allies against Japan, they came close to delcaring war on them but ultimately didn't.
-Republican exiles that could have toppled the government in a foreign-sponsored coup were very leftist and the US would not rather have a government full od communists that close to Western Europe.
-Franco didn't push international reclamations and usually commited to international agreements even if it took some convicing like in Equatorial Guinea.