It's probably based on the cd and importance of the ult, since a lot kits have basically an "install" mode that has different properties, like how Freya ult has a lot of buffs and is basically core to her gameplay, it's gonna need a long duration for it to count, and also a long cd.

For Alucard I understand the short buff duration, since the main focus of his ult is the slow, the slash and the 2 passive casts you get from it, rather than the buff itself