Another huge advance in number theory--alongside the subconvexity result posted last month--in a vastly different area of the subject. 2021 was a great year for number theory. (This of course assumes both results are correct!)
Maybe it was because for the last two years everyone was stuck at home because of Covid.
There are only so many Netflix movies/shows you can watch before you start playing around with number theory out of sheer boredom.
https://news.ycombinator.com/item?id=29790830 This one is also related to primes.