There is also so many fabric designs and companies to buy from and QSs want to keep their stores stocked with the newest lines. They may not be able to re-stock the fast sellers or they many not want to in favor of stocking another new line. The customers want to see "what's new" everytime they walk into a store.
It takes at least 3 months to print and ship a fabric line from overseas and this does not include concept/design time at the office. So, it is conceivable that it takes about 6 months total to produce one collection of fabric...from concept to printing, shipping, and on to the store shelf. Yes, buy it if you love it!
There are "exceptions" to the fabric reprint rule but it is not the norm. Some fabric companies do reprint lines but not all of them do or will. What is a "best seller" in one area of the country may not be a "best seller" in another...it even varies shop to shop.