There are lots of good meta-gamey reasons, but my characters don't know any of them. It's a nest of vicious, evil warriors who kill without a second thought, and who are looking for the very artifact you're carrying around - the one keeping your party sane. The extraction device turns out to be real, but it's deadly (no surprise there). And then Lae'zel wants us to start killing key personnel, at the risk of bringing the whole place down on our heads, when it seems obvious that the problem is her own cluelessness.
On top of all that, Lathander is my favorite FR god and it just makes me sad to see one of his monasteries in such a state. The only thing I would've really liked to do there is turn it back over to his clergy - not blow it up.
