primarily strength to weight ratio VS. metal alternatives... and while although relatively inexpensive in it's raw cloth state, it require's considerably specialized skill, processing/labor to render it into functional "load bearing" parts. that's where the cost comes in.
for example... like dymag wheels, helicopter doors, monocoque MTB frames, maclaren and ferrarri F1 chassis etc. etc. in addition, there's just some intangible "trick factor" about the stuff that allows producers to generally get away with charging us MORE $$$$ for it... LOL
it's actually very easy if ya wanted to do some stuff of your own. a buddy of mine actually made a solo tail for his R6 and another made his own exhaust hanger for his gixxer. and these 2 individuals have never struck as being the "sharpest knives in the drawer" if you know what i mean...?
while the stuff they made wasn't exactly "tops" in quality, fit and finish, it was good enough for government work... i could see though... if they wanted to put a lil' more time into it, their stuff would be easily as good as any others i've seen.
yes, they are DP and do have turn signal mounts. note, there's a bit of a crack (1/2") in the left side runner. due to the natural weave pattern, it's barely noticeable... but it is there nevertheless. i can't even get it to show up in photographs... LOL