From what I've learned from being in the fabric business for many years, there are several reasons why the fabrics come and go so quickly.
The fabric companies' sales reps always point out that when we (shoppers) walk into a store, we want to always see something new. I suppose this is true no matter what kind of store we're talking about! So the new fabric lines keep coming and coming.
That said... the fabric manufacturers can't keep everything in stock forever then, can they? So they sometimes re-print the most popular fabrics in a line a time (maybe two)... and then they drop the whole collection.
Did you know that most of the fabric companies show their lines to stores six months or more before the fabrics are actually available? They take pre-orders, and that way they know how much of each design to print. They would like it if, as soon as the fabrics arrive in their warehouse from the overseas mills, they immediately ship to stores. It's good for their cash flow if the fabric is all spoken for... so they don't print much more than what they've taken pre-orders for.
And if a magazine is printing a pattern using a given line of fabrics, they sometimes make that information available to stores ahead of time, and sometimes the fabric company will print more because they know the magazine will drive their sales too. Still... they're probably only going to do one extra printing to accommodate the demand.