Because 1984 was one of the three biggest electoral landslides in US history. Reagan won the popular vote 58.7% to 40.6% and garnered the biggest electoral college vote in US History. Mondale only won his home state of Minnesota by 0.18%. Reagan won Massachusetts (extraordinarily) by 2.8%, the next closest margin of victory in any state that year. Part of this was Reagan's affability, his projection of confidence, and his tremendous skills as a communicator/symbol and part of this was the recovery of the economy: after the rather sorry decade of the 1970s with scandals, poor economic performance, and slip-shod confidence emanatating from Washington, Reagan oozed confidence and prosperity. Many observers around the world noted that even most Europeans favored Reagan, because Mondale was talking about absurd promises vis-a-vis the Soviets [which ironically Reagan would probably have made given the chance in 1985]. It's a pretty good starting point.
What's more mutable is the piss-poor nature of the Democratic race that year. The Democrats had a large field of candidates, with no obvious front runners: this was the year of the most chaotic Democratic race...until 2008 came along. Jesse Jackson made a strong showing initially, but made some ethinc/racial gaffes; nevertheless, the party's primary election rules meant that even though he got 20% of the popular vote in the primaries, he only got about 8% of the delegates. The more heated race was between Carter's VP Walter Mondale and Colorado Senator Gary Hart. The latter had a lot of populist enthusiasm -- "new ideas" and the latter a lot of expereince, leading the epic line "Where's the beef?" Mondale ultimately won, but it was very close. It's part of why the changed the rules for the primaries to incorporate superdelegates. Oh, and John Glenn also made a go of it. The tight contest meant that the Democrats were low on enthusiasm and money while Reagan didn't have to campaign at all until the general.
Nonetheless, the dissarray in the Democratic party wasn't a new thing that year. It was a sign of the fraying of the old Democratic coalition, with "youth" / reformist progressives vying against old labor New Deal liberal against Southern / conservative values. It was also a legacy of Carter's run in 1976, which upset the balance of power as it were between these coalitions; and of Ted Kennedy's primary run against Carter in 1980.
None of the candidates running that year were charismatic, politic, or adept enough to garner wholesale Democratic support, unite the Party, and then reach out to myriads of voters who preferred Reagan. To change the nature of the race, you'd probably need a POD before 1980: Paul Volker not being appointed to the Fed, EMK dying of a heart attack and/or not running in 1980, Carter choosing a more charismatic running mate. Such a POD would / could change so much that's it isn't really changing the election alone, it's changing the whole game.