Why all manufacturers do not use composite?

Azdel may have 50% more R-value than Luan but you are increasing the value from R-0.25 to R-0.37 (actual values are lower). The only value with these synthetic materials is for the consumer--they are waterproof and weigh somewhat less, for the mfg--they are consistent and easy to tool/bond, they are a marketing ++.

I would suggest to the OP that maintenance regarding sealing from water is the best "science". If you develop leaks with composite materials the glue will fail and damage will be greater than a wood frame, i.e. delamination and rapid structural failure.