I think people who recently died get priority because they died fighting to defend Krakoa!
Behind them is probably the people in the Crucible!
Then mutants that have been dead for a while and require extensive counselling when they are brought back!
Of course we are forgetting the numbers they can resurrect per day is probably around a 10 to 20 mutants! So sure they tried to resurrect Kate 18 times probably on 18 different days spread out over a few weeks!
But they are still resurrecting other mutants at the same time! It's still a gradual process and it will take a while to bring back 16 million mutants!
It makes sense to resurrect the longer dead mutants very slowly, you have to make sure you have infrastructure for them, and more than anything counselling and support!