Legion is attacking all azeroth, not only broken isles. They might be somewhere in silithus battling infernals and boarding legion spaceships.From an official standpoint we've already been told (if I remember correctly) that Blizz simply didn't want to continue the stories of the Draenor characters and shelve them for now, because they've had their time in the lime light and now it's time for other characters. Which I agree with, don't get me wrong, I have at times drank heavily and incurred head injury to forget WoD happened, HOWEVER...
...Y'rel, at the end of HFC, specifically tells Dhadgar that Draenor will stand with us against the Legion, and then seemingly proceeds to shove us through the portal back to Azeroth and mutter "I can't believe he bought that lol" under her breath. Or so it would seem, anyway.
From a lore perspective, if Azeroth is saved from the Legion and the Legion is sufficiently buggered by us (IE we manage to kill Kil'jaeden in the Nether or even severely damage Sargeras, who knows what happens on Argus later) then by default, Draenor's future is now secure as well. Draenor gains a future it was never meant to have, it and everything on it literally rips a new destiny out of the jaws of damnation. You'd think we'd see some brown Orcs, some Draenor Draenei, and some Arakkoa NPCs running around lending a hand.
TL;DR I find it odd Draenor is "brb rebuilding or whatever" when all of that rebuilding is completely moot should Azeroth fall to the Legion; the Legion isn't going to forgive and forget Archimonde's defeat there. Would it not be wise to at least send something through to Azeroth to help?