It’s the difference between not agelessness and immortality.
Denarians, Vamps, sidhe are ageless, time doesn’t diminish them, but they can be killed with varying degrees of effort.
Immortals on the hand can’t (they don’t like to talk about Halloween) the Queen’s Mantles are constructs and had a back door built into it the Stone Table.
Aside from the Queens, the other Immortals we have seen are The Kringle Mantle, Erl King, the Naagloshii, Uriel, the Angel of Death, Ethnui, Hades, the Fallen. If the Darkhallow requires an immortal there aren’t that many roaming around these days, and some are just too powerful (the Mothers, Hades, Uriel, the Angel of Death, Ethnui, the Fallen) to even try to summon. The Erl King was probably the weakest and most accessible. The Ladies are ‘weak’ but are an artificial construct so would they work? Kemmler could have undertaken a Darkhallow using Maeve only to find to his horror he was the new Winter Lady subject to Winters rules, and being physically reshaped to the task.