Why indeed?
The target was known (specifically named) to high-level French authorities for over a year.
The killers were known to police and were on terror watch lists, one wore an electronic tracking device.
The church itself, if advised of these facts, could have had a volunteer, such as a concealed carry parishioner, in the pews at masses... oh, wait.
Not enough Sheep Dogs to keep the Muzzie Lone Wolf Packs at bay.