Walter Borneman (Polk: The Man Who Transformed the Presidency and America) claims that it was a shrewd political tactic to get his party to unite behind him.

Polk's nomination was the result of a compromise at the 1844 democratic convention. Initially, the main front runner was former President Martin Van Buren, who had been defeated by William Henry Harrison in 1840. However, his views on expansion (he had turned against annexing Texas, mainly due to concerns about the expansion of slavery) mad him unpalatable to much of the Southern Democratic establishment. Polk initially planned to seek the Vice Presidency, providing regional balance for Van Buren (a New Yorker). However, as it became clear that Van Buren's views on expansion put him out of step with the party, Polk's friends (candidates did not generally attend conventions) put him forward as a compromise candidate.

Due to its divided status, the Democratic party had no shortage of alternative presidential contenders (including James Buchanan, and John C Calhoun). Polk believed that by promising to serve one term he could unite his these competitors behind him, where otherwise they might scheme to unseat him in four years. They would have no reason to sabotage him if they did not have to run against him in the future.

Additionally, the precedent at the time was for presidential candidates to feign disinterest in the office, framing a run as an obligation to party and country rather than taken out of personal ambition. Due to this, had Polk decided to seek a second term after all, he had a ready-made excuse 'I planned to only serve one term, but the people compel me to remain' or some-such. As to why he didn't, he set out a series of goals and accomplished them, retiring 'on top,' rather than attempt another term.