My guess would be because skin cancer rates fluctuate depending on one's location within a country and in the world, whereas, in general, sex-specific and lung cancer rates are comparable with respect to intra- and international locations.
Therefore, the mentioning of skin cancer is implicit of the fact that there are some locations where it is way more prevalent than other cancers.
Coincidentally, I'm currently living in Brisbane, Australia, and skin cancer rates here are higher than anywhere else in the world (even higher than in the northernmost of Australia), and just about every elderly Caucasian person's got an actinic keratosis or some sort of pearly, telangiectasic lesion.
SPF50, vitamin E and wide-brimmed hats!