Well, I went to Ask.com and poked the question above into the search field, and was rewarded with the answer from ProperAutocare.com, which explained that ozone and ultraviolet light from sunlight attack the polymers in the tire:
To protect against ozone and UV damage, a stabilizer molecule called a “competitive absorber” is blended with the tire polymer. Competitive absorbers work by capturing and absorbing UV radiation and converting it to heat which is dissipated harmlessly. All tire manufacturers use the same competitive absorber, carbon black. This is why all tires are black...why tires are not available in designer colors.How did we get along before the Internet came along to answer all of our trivial questions? The answer: Not as well as we do now.