I'm assuming the "Death" refers to the fact that it deals Necrotic damage. And yes the damage isn't as high as other spells at 6th level, but the radius is HUGE. It's got a 60' radius, compared to only 20' for a Fireball. Circle Of Death may not be great for small groups of heavies, but for clearing massive hordes it can be very impressive.
Yeah I don't think this can be understated. A 60' radius spell hits
over four hundred 5x5 squares. (Technically, it hits 452 squares, but I'm allowing for all the inefficiencies and "must cover at least 50%" rules which cause a decent amount of loss at the edges.) Compare that to a
fireball, which covers an ideal area of 50ish squares. Are there a lot of situations where you
need 8x-9x the area of a Fireball spell? Probably not. But I can at least see some benefit for stomping on such a large area.
Circle of Death, really, is a bad guy spell. For commoners and guardsmen, having only 1-3 HD apiece, 28 damage pretty much IS death. So what if it's sub par for most nasties PCs will face? It's GREAT for a cackling madboy wizard delighting in the suffering of innocents.
That said, it also has
some uses in dealing with a huge enemy onslaught. Remember that 5e expects you to continue using CR 1, 1/2, or even 1/4 creatures at 12th level and beyond; for many such creatures, while this spell might not be instant death or even a killing blow, it'll hurt like hell and just one more hit will take out a lot of things (and at this point, Fighters are getting a third attack, and most other melee classes hit extra hard with theirs, so they're quite good at this sort of 'cleanup').