Because priests require warrank and money.
I beleive you're paying for their ability to heal and do some minor ranged DPS, not act as tanks. Also, I think their healing is meant to keep the creep tanks alive so that they can do some DPS, but it doesn't seem to be working out, since the minotaurs seem to die almost instantly... again, in the end you just get a giant line of priests healing themselves whenever they take damage, until you go in yourself and kill them all.
Also, upgrading minotaurs to 510hp and 400 armor (which prevents about 15% damage I think) costs 9600g and war rank 8... getting priests with 589hp costs 1800g at war rank 3, and they'll have 765hp if you spend the 9.6k in upgrades.