That might have more to do with a respect for the fact that no matter what, we are still modern humans. Diplomacy is something we all more or less understand and take as granted in the game. Religion? Not so much. For evidence you only need to find how many people still insist that they are atheists in the game.
Now that I think about it, it makes sense. A lot of the incongruities in the game that differ from a true Medieval recreation comes from the fact that we are modern humans. In many of our countries, religion simply does not play a major role in the government. The United States has the separation of church and state, China does not endorse any state religion, and I'm sure those more familiar with different countries' systems would know more examples.
In-game, this means that you can see how many realms say they have no state religion either. PoZ, last I heard, said they would allow freedom of religion, and furthermore that it would mean no religion could get a foothold on the realm. Nothoi also practices freedom of religion. To name realms that actually "matter", even Sirion, surprisingly enough, does not have a state religion, in that lords are still free to choose their faith and their regions' faith. It just so happens that one religion is more prevalent, but does not appear to interfere with the political side.
Besides, I've noticed that Chenier seems to like more realism in religion. But above all else, this is a game, and we could make the diplomat class less time intensive. Making the religions more realistic might not actually work in this game, since the majority of interactions should be roleplayed, and it doesn't sound like that many people are really into it except maybe for the rare exceptions. Ok? So? We have only a handful of major religions in this world, and the top three all happen to be monotheistic and originate from the Middle East.
Now, we do have a system to make priests more important. They are called theocracies, but beyond that, it's up to you to get support to have your power. That's what real power is, no matter where you go or what you are.