Because some religions can be more introspective than others. I think that Loss can come in many ways and shape a character, and that's important when it comes to his religion. I think this is confirmed by the secrecy of the religion and why many things are hidden from non-followers.

There are also many ways you can bring loss or reveal it to non-followers. I think it's quite possible that Rahsael's religion can bring about Loss in many ways. The issue is that players choose the easiest way to "preach it." Blah blah let your loss make you stronger blah blah blah. That's the easiest way to go from point A to B, so many people take it.

If there's any fault to the religion, I'd say that it's one where there is a dominant theme that drowns out everything else. For instance, let's use Twist's religion since it's the first that comes to mind and I had followers in the past. You can go the pure magic sphere, but you could also go honesty sphere and follow Twist with a completely different concept.