Three main reasons… The awnings sold on the internet are made mostly from (1) painted steel (2) inferior fabrics that will fade quickly and (3) cheap Chinese motors. The steel components and steel hardware of the frame will begin to rust after just one season. The fabric will also noticeably fade after just one season in the sun. Many times the motors do not work right out of the box or the cheap transmitters and electronics fail quickly.
Awnings sold by your local awning shops are made mostly from powder coated aluminum and stainless steel which will not rust. Awning shops use acrylic fabrics designed specifically for the awning and marine markets. These fabrics will remain weatherproof, are mildew resistant and hold their color for up to 20 years. Better awning companies insist on Somfy brand motors for all retractable awning products. Shade One Awnings only uses Somfy as they are the clear market leader. These motors can also last up to 20 years.
Beware of local companies that advertise themselves as “warehouse” or “manufacturer” awning companies. They provide low cost awnings that often fail very quickly for the reasons listed above. Many times Shade One Awnings is the company that is called on to fix or replace these lesser models.