Found the following bit of lore in a random google search:
"When they first appeared, kirlanans were outside divine influence and hated all deities. However, they formed temporary alliances with the servants of wicked divinities, mainly Shar. Many kir-lanans liked Shar’s doctrine of ultimate dissolution. Over time, the race fell under Shar’s sway."
Not sure if it is an official explanation or not, but seems to make sense. They see Shar as the best route to the end of the all the gods - and that may be so. But whether it is or not, enough time working for Shar, and they are tied too tightly to ever let go...
