I guess because they have more "parts." A pair of panties is just some fabric with elastic, maybe a bow or something. Bras have wire, padding, fasteners, adjusters... lots of parts.
 
I agree, though. They are WAY too expensive! And VS are the only ones I like too. Granted, I haven't tried many other brands, but these work better for me than really cheap ones, so why not?
 
I am pregnant and I'll need new bras soon. I'm NOT looking forward to that shopping trip.