Priest mobility is terrible because Blizzard wants every class to have different strengths and weaknesses but Blizzard’s design is so uneven that those strengths and weaknesses are way out of alignment.
Somebody needs to be the slowest class, but Blizzard has been creeping up mobility dramatically and every class except priest has gained mobility and numerous ways to resist or remove slows/roots/forced movement/crowd control within the last twelve years. Priest has only lost speed in the last ten years, gained none of the slow/root/cc resistance the other slow classes have, and has no notable strengths that compensate for that massive weakness.