My take, based on CNN's exit poll, is that, as is often the case in non-presidential election years, the election is determined by who actually votes.
There were about 1.1 million fewer voters in the Virginia election than in 2020, and under 30 voters accounted for almost exactly half of the decrease (about 325,000 vs 875,000 in 2020). There were also around 600,000 fewer members of minority groups who voted, a decrease of around 40 percent. On the other hand, there was actually a small increase in the number of over 65 voters. So 2 groups that lean solidly Democratic didn't show up at the polls while an important component of the Republican base did (over 65's were more than 3 times as likely to vote as under 30's).
The CNN exit poll showed only about 5 percent of Biden voters voting for Youngkin, which I don't see as a particularly significant swing, given that the 2020 election involved one of the most polarizing figures in American political history. (2 percent of Trump voters voted for McAuliffe.)
IMHO, McAuliffe ran a poor campaign, letting Youngkin get away with not revealing whether he would take any steps to eliminate abortions or what programs he would cut if he went through with his plans for cutting taxes while increasing spending on k-12 education.
