Okay, lets think about this logically...
HH is not available on steam right? Nor is it available on Direct 2 Drive.
Surely that means that its a problem on Bethesda's end and not steams.
I can't say I've ever really heard of Direct 2 Drive before this/these threads so i'm sorry if what i'm saying is a complete load of rubbish.
D2D is just another digital delivery service. But in this case HH is a Steam exclusive. And Bethesda has already said (via Twitter, apparenlty) that they are waiting on Steam, so that implies its really just a Steam issue.
Like others have speculated, it probably has something to do with two major PC titles coming out today and Steam prioritizing those in terms of getting them up and making sure their servers can handle the load.