I think part of is the difference between how Valve and Blizzard structure their respective esports scenes. There's a ton of variety in majors/tournaments in CSGO that require a site like HLTV to easily follow the scene, whereas Overwatch has just... OWL pretty much (which happens to have a half year offseason for some reason).
Right now the entire front page of hltv is just shitposts or people saying players should be kicked from teams.
but yea CS has tournaments almost 365 days a year. OW we have weeks with no news or matches.
Because they are different websites that talk about different games frequented by different people talking about different things
Over.gg just hasn't caught on within the general OW community. Unlike HLTV and even TFTV, this site isn't known by everyone in the community.