lol H1Z1 was VERY successful for awhile, but every game has a limited life span, especially when something new & better comes along. Fortnite & PUBG will suffer the same fate eventually too.
H1Z1 was never "very successful", which is why Daybreak Games split H1Z1 into two separate games, H1Z1: Just Survive and H1Z1: King of the Hill and recently rebranded H1Z1: Just Survive by dropping H1Z1 from the name and renamed the game to Just Survive. While H1Z1: King of the Hill has seen some success, it was not enough to stave off DB Games from laying off most of the H1Z1 team members because the game isn't meeting expectations. DB Games is putting in a lot of hope that the H1Z1 pro league will change things around.