Valve sends a few nominations every year, based on player rankings I think. And last year, some other player in some other sport did something spectacular lol. CS might require the highest skills, but Cod/Fortnite/LoL/PuBG players gain more popularity nowadays. That's why they win.
CSGo players won in 2015, 2016, when CSGo was the most popular game by a huge margin. In 2017, somebody else won it. In 2018, s1mple brought in huge viewership and gained literally millions of fans, several millions of votes, and won the award, also made CSGo the most popular again. In 2019, neither s1mple nor Zywoo were the most popular in all of esports :(