Are glider winch launches rarer in the USA than in the rest of the world? Why?
To expand on Quiet Flyer's answer, it's technical and economic at the same time.

Technical: Winch is limiting because you are deposited in the same spot over the field, whereas a tow, if the pilot knows what he's doing, can take you to a thermal up to a few miles from the field.  Plus you can get towed higher.  If you had to choose the launch method that gives the highest probability for a successful flight, aerotow's a no-brainer.

Economic: Because of the higher average individual purchasing power in the US (and Canada to more or less the same degree) of disposable income (for example, power flying is a middle class, even lower middle class, activity in the US and Canada, and an upper middle class activity in EU), glider pilots can afford the higher cost of a tow, but which is still cheaper than a tow in Europe because overall operating costs for power planes are less.

So given the choice between paying 15-20 bucks for a winch launch to hopefully 2000 ft (if you have a runway long enough to fit the cable) and 30-40 bucks to be hauled to 3000 feet in a Pawnee, whose pilot, if reasonably skilled, will find and drop you off right at a thermal, most North American glider pilots will pay the 30-40 for the tow.

So the result is there is very little market demand for winch operations in North America, except with some small clubs in out of the way places without the money and resources to run a tow plane.