Why wasn't it made to ably to skeletons, and other undead creature, or creature whose remains we are excavating, why just demons?
Could there be a future addition, to add undead creatures, giving us a resemblance of necromancy?
As others have said, the site story explains why, but perhaps you've missed this part... The entire purpose of that whole site is to achieve this aim (the gate connects to Dis, the capital of Pandemonium, on Infernus). Necromantic summons (if we ever make them and they come from Summoning) would need to come from a different 'source'.