Why did Ford pardon Nixon?

There is no clear answer to this question, as President Ford did not give one before his death. Many speculate that President Nixon made a deal with Ford, stating that he would resign the presidency, allowing Ford to assume the office, with the condition that Ford would pardon Nixon. Ford made Nixon sweat, and did not pardon him immediately.

There is no direct evidence, however, that such a deal was made. Perhaps Ford felt it was the right thing to do, given that Nixon was made to take the entire fall for the Watergate scandal. Or perhaps Ford still held friendly regard for his one-time friend and colleague. Unfortunately, nobody will ever have a definite answer, since both parties are dead.

Conrad Black describes the circumstances in Richard M. Nixon: A Life in Full as follows:

The inevitable swarms of conspiracy theorists claim that [Alexander] Haig brokered a pardon for Nixon from Ford. Both Haig and Ford deny this and have done so in identical and strenuous terms for over thirty years at the time of writing ... Further, Nixon considered himself a wronged and tormented man; he was not seeking anything that would imply admission that he had done anything that justified the present legal condition ...

At Ford's first presidential press conference, on August 28 [1974], there was a question about a possible pardon of Nixon, which Ford parried. Hugh Scott and [Nelson] Rockefeller had both said publicly that Nixon had endured enough and should not be pursued further. Ford said that he agreed with Scott and Rockefeller, but that there was no judicial process under way and he thought it inappropriate to comment further. The press took this to mean that Ford would pardon Nixon after a trial but not before ...

In Washington, Haig had spoken to Nixon and been bombarded with calls from his daughters and sons-in-law expressing concern about Nixon's health and morale. David Eisenhower called President Ford on August 28 and made the same point with him. [Leon] Jaworski advised Ford that he was not planning to ask for an early indictment against Nixon, but a grand jury might prefer one, and that it would take at least nine months to get a trial started. No one seriously thought it would be possible to empanel an impartial jury anywhere in the United States in such a case, and the timetable Jarkowski outlined would have the trial of the former president rolling into and trough the election year of 1976.

Ford told his counsel, Philip Buchen, to tell Nixon's new lawyer ... that he was considering a pardon, but that he wanted a statement from Nixon that would be an act of contrition ... There were four drafts, mainly composed by Nixon, who refused to acknowledge any guilt, but was prepared to express some remorse ...

[Benton] Becker finally requested to see Nixon, so he could report to Ford on his condition. He found the ex-president shockingly diminished in the month since he had left Washington. He was jowly, pallid, almost shrunken, and had a limp handshake and a distracted manner. Becker reported to Ford that Nixon was severely depressed and he doubted if he would life more than another couple of months.

On Sunday, September 8, Ford went on television on radio, explained that he wished to put Watergate behind the country and the terrible divisions it had created, and read his proclamation of a "full, free, and absolute" pardon for Nixon.

Hope this helps.

One doesn't need to speculate, he specifically stated the reason why in Proclamation 4311:

It is believed that a trial of Richard Nixon, if it became necessary, could not fairly begin until a year or more has elapsed. In the meantime, the tranquility to which this nation has been restored by the events of recent weeks could be irreparably lost by the prospects of bringing to trial a former President of the United States. The prospects of such trial will cause prolonged and divisive debate over the propriety of exposing to further punishment and degradation a man who has already paid the unprecedented penalty of relinquishing the highest elective office of the United States.