Because the fact players primarily have to either conduct Raid or Mythic+ (for PvE) to obtain gear has been the primary two avenues for PvE and with Delves, players are ENJOYING the relief of not being “forced” to participate in hours worth of Raid or the headaches of Mythic+.
Delve rewards cap out to item level 616 from T8-T11 via the Great Vault. By Blizzard not increasing rewards beyond Delve Tier 8 is a GOOD thing so it does not 100% replace Mythic+8 and beyond Great Vault reward to a maximum of 623.
Right now, for players experiencing Delves vs Mythic 0 “rewards”, Delves is more ENJOYABLE to conduct either SOLO or GROUP over the pain with M0 (soon to be Mythic+) pugs for majority of players. I don’t know about you, but Delves relieve the headache of Mythic+ and you know, it’s a good thing for players to take a break from Mythic+ and concentrate on pushing themselves through Delve tiers. And boy, let me tell you, the Season 1 Mythic 0 week has been a infuriating experience. You seem to forget, not everyone played Cata or BFA with the Seasonal dungeons added to TWW S1, so, there’s that pain of people “not knowing” mechanics ect. So, going straight into M0 with ZERO knowledge or experience is frustrating, and yes, I know, it’s nothing new, but players would rather not deal with it as much as people will have anyway.
This is the Diablo effect with the way the Delves are to be honest. If you don’t know what I’m talking about, Infernal Hordes in Season 5 is exactly that, a Delve. The higher you go, the more “risk / reward” you have.
Say what you want, I’m not hating on Mythic+, I’ve enjoyed Mythic+ on ALL of my toons since Legion and with the introduction to Delves, this a breath of fresh air for those of us who keep putting up with countless wipes, bricked keys and leavers in Mythic+ whereas you can grab a team of 3 or 5 in Delves, without a timer, good loot, good companion experience, good rewards at the end of the Delve and within the Great Vault. Yes, I AM a Delve enjoyer and I HOPE these Delves are here to stay and more of Delve content is released in the near future!