PUBG's popularity is a phenomenon. Developed with a small budget, it started at humble beginnings with a dream of surpassing 100,000 concurrent players. Just five months later it's breaking 800,000 concurrent players, and hitting the top spot on Twitch with hundreds of thousands of viewers.
As PUBG has grown, some other PC games have faced casualties. As noted by Steam Spy, a large portion of the playerbase has come from Counter-Strike: Global Offensive. It stated:
55% of PUBG owners are CS:GO players. They used to play CS:GO 50% more than an average person, now they play 30% less.
It added:
PUBG is pulling the most engaged players of CS:GO and H1Z1. It also pulls the least engaged of Payday 2 and Left 4 Dead 2.
This trend has been visible on Twitch, where several of CS: GO's most popular streamers have transitioned to PUBG. Perhaps the biggest example of this is former Cloud 9 player Michael "shroud" Grzesiek, who after three years of competing in CS: GO's professional scene and making over $200,000 in tournament winnings has become a full-time streamer, with PUBG being his primary game of choice.
PUBG and CS: GO are very different in their design, but both feature a heavy emphasis of teamwork and competition.