Where are the cloth / fabric minifigure capes / cloaks in my set?
I found them.

The capes were packaged in a small, unmarked, rectangular, white, cardboard box that was taped shut. Apparently, this little box came with the set, but someone else assumed it was filler trash and put it on our recycling queue.

Considering the capes are the only items out of stock besides the Cave Troll, it is possible others may be simply dismissing the box, and discarding it as filler.  This could explain why they are out of stock (or, as TheBrickBlogger's answer indicates, they are popular items like the Cave Troll).

After discovering the capes, I've cancelled my replacement order.  I provided feedback to LEGO recommending they make the container box more obvious.