There were a number of factors behind Nixon's victory in 1972. But I wouldn't give the entire credit to Nixon's own efforts or the inherent advantages of his incumbent status. I tend to think that a major factor behind Nixon's victory, or at least the sheer size of his victory, was that the Democratic Party was essentially breaking apart. Now of course, my conclusion may be somewhat simplistic. But the Democrats were clearly suffering from a broken base. The traditional or "Humphrey" Democrats were not willing to support someone like McGovern, and conversely, McGovern's supporters would never support a Vietnam War supporter of any stripe. The broken base probably dates back at least to the previous election. I'm not discounting Nixon's influence here, but the split between the antiwar crowd and the traditional Democratic vote that he exploited in 1972 was not entirely his doing. If the Democrats want to do better, they need something of a unity candidate. Someone who appeals to the traditional Democratic base and the antiwar crowd. Problem is, that candidate simply did not exist.
Sorry if my answer seems rather glib, as it has been a few years since I've studied the 1972 election, and when I did study it, it was almost entirely from the Democratic perspective. As such, my viewpoint is rather skewed.