I hope that it could be understood from the text that not all the priests were necromancers )))

But the main reason that they mainly were part of church is education. In the Middle Ages almost nobody could read except for clerics and sometimes nobles. It's just the time when first universities appear in Europe. The rituals were closely connected with knowing Latin.

I make these notes since I'm studying now the course Magic in the Middle Ages by University of Barselona https://www.coursera.org/learn/magic-middle-ages
And the main reason for studying was that everything we think about magic in those times goes from movies and fiction :)