Originally posted by: Zenmervolt
It's a shift in societal conventions. Tanning represents affluence, either the ability to vacation in places where one can get a tan (and the requisite income that allows those vacations) or at the very least it shows that one has sufficient income to "waste" some of the income on tanning.
This is distinct from past periods when pale skin was considered to be the ideal as only the aristocracy could afford to keep out of the sun. Common folk had to labor outside and ended up with tanned skin, which was then associated with a lower-class status while being fair-complected denoted superior birth.
With the advent of the modern industrial economy that no longer needs the majority of its workers outdoors in the sun, the standard of beauty shifted from pale skin (since now almost everyone worked indoors and tans became uncommon) to being tan (since only people who are well-off now can afford to be tan).
Of course, if you were to ask a random person off the street, they'd just say that they think it looks "better". Most of the time these sorts of social mores are not consciously apparent.
ZV