It is a combination of him being an extremely gifted politician, as well as the historical situation during and leading up to each of his campaigns. In the 1980s, the southern states had transitioned to the Republican Party, whereas they had been mostly democratic since the Civil War (Lincoln was the first Republican President). That changed with the Democratic passage of the Civil Rights Act in 1964. In addition, in the 1980s, California was still a reliably Republican state, dating back to 1952. Meanwhile, President Carter was quite unpopular, due to a prolonged recession, high gas prices, double digit inflation, and the iran hostage crisis (which lasted during the entire election). In addition, Ted Kennedy (similar to Bernie Sanders) mobilized the more liberal wing of the Democratic party against Carter during the primary, and their support of Carter was tepid at best in the general election. Meanwhile, Regan had a grand plan to fix the economy, and was able to make promises of change. He also had a demeanor that engendered optimism, and was extremely effective at communicating his plan. The first election was as much a repudiation of Carter as it was a huge endorsement for Reagan. Reagan won with 52% of the vote, but had 10% more vote than Carter due to a liberal Republican taking about 7% of the vote. The result was a landslide electoral college victory.
The reelection campaign benefited from the demographics of the electorate described above, but also from the fact that things did improve (at least in the short term) during Reagan's first term. Inflation greatly slowed, the economy growth rate picked up, the hostage crisis ended shortly after Reagan took over, and when Mondale began to gain some traction after the first debate, Reagan performed extremely well in the second debate, essentially stopping any momentum Mondale had. The result was one of the biggest landslides in US history.