There are several factors that, taken together, may explain this spectacular outcome.
The first is experience. Back in 2016, Sandu was still in the middle of her transition from a successful technocrat to a politician of national caliber. Dodon was also a technocrat, but made the transition to politics much earlier. In terms of political games and communication, he had the edge. Sandu spoke like a project manager; Dodon acted like a savvy populist. Sandu stayed away from toxic politicians; Dodon has been less selective in choosing political allies to further his political ambitions. However, after a period of political reconnaissance, Sandu learned how to talk and to practice political elbowing without losing credibility. It was Sandu who pushed for a tactical coalition with Dodon in 2019 to dislodge Plahotniuc, whose grip on Moldovan politics had become dangerously tight. At the same time, as prime minister she did not cling to power at any cost. Sandu kept pushing for an anti-corruption agenda, which was a no-go for Dodon who, after five months, decided to pull the plug on the alliance. Overall, though not without bruises, Sandu emerged stronger and better prepared for a brutal election campaign.
The second factor is party support. In 2016, Sandu's political project, Party of Action and Solidarity (PAS), was still in its infancy and short of funds, with few territorial structures and facing constant harassment of its supporters by law enforcement bodies. Dodon was backed by the formidable political machine of the Party of Socialists of the Republic of Moldova (PSRM), which had cannibalized the local structures of the former hegemonic Communist Party of Moldova. By 2020, PAS had become a veritable political force, too, with mayors, local councillors (as a result of the 2019 local elections), and functional branches across the country. In terms of infrastructure and resources, it remained vastly inferior to PSRM, but still, Sandu had much stronger and better organized support on the ground than four years ago.
The third factor is perception. The images of both candidates had undergone a dramatic shift in the four years since the last election. Sandu managed to add to her unblemished integrity (a very rare quality in the post-Soviet region) the gravitas of a stateswoman who has experience in managing the country and representing it internationally. She was also able to rekindle hope among Moldovans: not an easy task in a country where the population is chronically disappointed in its political class. Dodon's image, meanwhile, rapidly sank. His unnecessarily numerous visits to Moscow created the impression in the public opinion of a Russian puppet. The overdose of overt Russian support (and also covert support exposed by media) backfired. Dodon's informal alliance with Plahotniuc came back to bite him, too. A video of Dodon receiving a plastic bag allegedly containing money from the tycoon became a viral meme and the source of multiple jokes. Kuliok (a plastic bag) became the president's derogatory nickname, and it is probably the word of the year in Moldova. This was devastating for Dodon's image and popular standing. On top of all this, the outgoing president had no major achievements to show: only a mismanaged coronavirus epidemic and the looming economic storm.
The fourth factor is competition in the populists' camp. While in 2016 Dodon was populist-in-chief of the pro-Russian flank, things changed dramatically when Renato Usatii, the leader of Our Party (which was excluded from parliamentary elections in 2014), returned from Moscow. With formidable communication skills, more refined populism wreathed in anti-corruption discourse, and the simultaneous use of the Russian and Romanian languages, Usatii successfully chipped away at support for Dodon. In the first round, Usatii got an impressive 16.9 percent; in the diaspora vote he came first in Russia (Dodon's traditional turf) and second in Europe. This partially explains Dodon's underperformance in the first round. In between the two rounds, Usatii used his growing presence on social media to keep pounding Dodon. Ultimately, he called on his supporters to vote against Dodon, and some of them did, contributing to Sandu's big win.
Last but certainly not least — in fact, probably the most important factor — is the diaspora. Moldovans living abroad vote overwhelmingly for pro-European parties and candidates. They also turn out in large numbers to exercise their right to vote. In 2016, 67,000 members of the diaspora voted in the first round of the election, followed by almost 139,000 in the second. The upward trend is therefore nothing new. The first round in the 2020 election two weeks ago only confirmed this ascendant tendency, with 149,000 votes cast outside Moldova. This diaspora activism unnerved Dodon, who described Moldovans living and working abroad as a parallel electorate whose agenda and preferences are in stark dissonance with those living in Moldova. This description, voiced between the two rounds, caused an uproar among the diaspora. Migrants sent home a sum equal to 16 percent of Moldova's GDP in 2019, making the diaspora the biggest "foreign investor" in Moldova. Voters living abroad responded to the president's remarks at the ballot boxes, and this proved fatal to Dodon's chances of reelection. In the second round, 262,000 people voted abroad, and 92 percent of them chose Sandu, ensuring an electoral slam dunk. This record participation by the diaspora offset the effect of organized voting of (31,000) Moldovan nationals from the breakaway region of Transnistria, who voted overwhelmingly for Dodon. The diaspora accounted for 16 percent of all voters in this election and became a force to be reckoned with.