The race was decided by a margin of about 55,000 votes in Kemp's favor.
Kemp, as Secretary of State, was in charge of Georgia state elections, which presented a conflict of interest for him as he sought higher office. Kemp implemented a voter suppression policy called "exact match," which helped to severely complicate the voting process for about 53,000 Georgians, 80% of whom were people of color.
More info: https://www.washingtonpost.com/opinions/georgias-voter-suppr...
Also, voter fraud basically does not exist in the United States as per this article from the Brennan Center: https://www.brennancenter.org/analysis/debunking-voter-fraud...
In fact, the most notable example of election fraud in the United States in recent memory is in NC's 9th Congressional District, where it looks like the Republican in the race cheated his way to victory: https://en.wikipedia.org/wiki/2018_North_Carolina%27s_9th_co...
edit: as burlesona rightly points out below, the article to which this comment is attached is about the Lt. Gov.'s race. My comment is about the general lack of integrity in Georgia's elections, and the deeply undemocratic (small-D) results that emerge from this state of affairs.