There are a number of reasons why President Nixon won reelection in such a huge landslide in 1972. What the reasons boil down to, though, are that Nixon was tremendously popular and his opponent seemed like an extremist.
In 1972, Nixon was quite popular. His record to that point seemed very good to most Americans. He was getting the US out of the war in Vietnam, a war which had become very unpopular by this point. He had established diplomatic relations with China. This was a major event and Nixon got the credit for it. The economy was doing fairly well. In such circumstances, it would be very typical for an incumbent to win.
On top of all these advantages, Nixon had help. The Democrats were in disarray. They ended up nominating a very weak candidate in George McGovern. McGovern was seen as too radical in both his domestic and foreign policies. Adding to that, he picked a vice presidential candidate, Thomas Eagleton, who turned out to have undergone electric shock treatment for depression. McGovern emphatically backed Eagleton when the news came out, only to dump him shortly thereafter. In other words, McGovern was a very bad candidate.
With a good record of his own and a weak opponent, Nixon won in a huge landslide.