[rdonj]

Well, I can't say for sure it's the reason but holy has a pretty huge power curve. A h1 priest is nearly useless, h2 starts providing tangible benefits, h3 priests can bless entire armies by themself and throw attack spells, and h4 priests can embolden every unit on the battlefield. Since you don't need any research for priest spells, the only thing that limits a priest is their level. I guess another reason would be that a priest has their place in the pantheon of the faith, and you don't really get especially powerful priests - every priest of the same level is viewed with the same level of holy authority.