Because cyclists waste hundreds of dollars each year on cycling-specific accessories when non-cycling-geared materials would suit just fine (or often, much better).
Other cyclists waste hundreds of dollars each year on non-cycling-specific accessories, and then have to spend the money all over again when they learn how wrong they were.
I wear my summer-weight gloves through sub-freezing temperatures. The main thing that freezes fingers is wind. Block it (and rain) before it even gets to your gloves:
http://www.trails-edge.com/retail/te...mfbikemits.htm
Those are Moose Mitts. They mount on your handlebars and cover all the stuff you need to touch (brake levers, shift levers, grips, etc.). Your gloved hands go inside them.
I wear crocheted half-gloves down to around freezing, and then summer-weight full-fingered gloves below that. Nice and comfy inside the Moose Mitts, and no fumbling around with thick fingers.