Because at ilevel 680 mythic dungeons are an actual challenge. Just getting a group can be an absolute pain in the ass. There's a reason why people demand 720 ilevel for mythic dungeons. Because that's the ilevel required to run them without having to care about the mechanics. Prior to that you need to actually care, learn, understand.

And before you go "Mythics are super easy bla bla" yeah we get it, you're an amazing player and mythics are so easy for you. But for the vast majority of the playerbase mythics are quite challenging prior to outgearing them. PvP is unbelievable easy by comparison.

If you can't see the difference between running around in Ashran and go from ilevel 630-700 in an afternoon to running mythics with a PUG at similar ilevels then you're either the single greatest player on the planet or you've only ever run them after you outgeared them. There's not even a debate here, Ashran is the single easiest and fastest way for a new or non-elite player to gear up for HFC.