Did the church run out of Exorcists?
So the church acknowledges this new demonic threat and decides to send a lone nun for investigating it? Very uncharacteristic of movie-world catholic church to not atleast send some priest along.
Sure, they say she is the only one who has faced this kind of thing.. Well then they might strap her to the back of their chief exorcist as a good luck charm and send them on their way..
I assume the producers didn't want Irene to walk in the shadow of someone, but then it would have made more sense with a plot where the church ignores this threat and she goes at it anyway, to save Maurice