Hmmm... apparently there are a few reasons mixed in with a healthy dose of scaremongering... (Fastcompany):
If aging faster isn’t scary enough, it turns out that frequent business travelers are exposed to more radiation than is considered healthy. "Radiation exposure is hundreds of times higher at high altitude than at ground," says Cohen.
Matter of fact, it’s so high that "there have been calls to classify frequent business travelers as ‘radiation workers,'" he says, and notes that just seven round-trip flights a year from New York to Tokyo (about 85,000 miles) exceeds the limit for public exposure to radiation. As Cohen notes in his paper, "radiation exposure amongst commercial aircrew even exceeds that of nuclear power workers."