Starbucks sells coffee, ambience, “a cosy living room,” and that’s its secret, you may think.  But consider this:

Starbucks sells milk. The Frappuccino, which Clark calls a “glorified milkshake”, is the product that catapulted the company into market domination. The secret of Starbucks’ success is that Americans love their dairy.

That’s Jakob Norberg in Times Literary Supplement for 2/29/08, reviewing Taylor Clark’s Starbucked: A double tall tale of caffeine, commerce, and culture. 

So.  Milk sells.

Another reason I have read somewhere for S’buck’s success is its marketing idea that you come in for coffee and are encouraged to hang around.

This author lays out complaints against the company but ends in defusing them.

For instance: while there are many stories of Starbucks’ fierce business practices, there is little evidence that it pushes people out of the market. If anything, it has inspired a greater interest in quality coffee and, as an unintended effect, the number of independent coffee houses has increased greatly.

So.  The better mousetrap led to imitators trying to better the better.  We call it competition.