One of the easies way would be to make diamonds a bit rarer.
(They were all used up with those thousands of ressurections in the past)

And since raise dead, resurection and true ressurection needs lots of diamonds, they have the trouble finding enough of those.
(probably there is enough in the lair of ancient dragons, or a lot of smaller ones, depending on what makes sense at the current point in the campaign)


(For reincarnate you can say the same, that they have to questto find those rare oils and unguents)