First, most beer sold in North America is pasteurized, and the plastic bottles commonly used for soft drinks can't withstand the heat of that process. (In the United Kingdom, where most beers are not pasteurized, several breweries do indeed market beer in plastic bottles.) Although more durable plastic bottles could be manufactured, it would be cost prohibitive -- would you pay $20 for a six-pack?
Second, breweries want as long a shelf life for their product as possible. Plastic is far more porous than glass, and carbon dioxide can diffuse through plastic and escape into the air, while oxygen can penetrate the plastic container, resulting in a flat beverage. Also, the dark and semi-opaque glass bottles in which many beers are sold helps protect the beer from sunlight, which can result in the dreaded "skunky" taste.
Finally, many companies are reluctant to market their alcoholic beverages in plastic containers because most consumers seem to associate plastic bottles with adolescent products, while glass bottles are seen as more "adult." Still no word yet on when they're going to make an alcoholic beverage that will actually make you appear more attractive and witty to the opposite sex. *