Most Helpful Guy
In the olden days, people loved pale skin becauae it was a sign of wealth. You spent every minute shaded from the sun because you could afford to have umbrellas, carriages, lounges and whatnot.
BUT on a more primal level, a pale face looks sickly. Males are instinctively attracted to females who look like they will give birth well, so rosy cheeks will turn them on. Fun fact, as you approach your time of the month, your cheeks will automatically get more color, and your face will actually change slightly to look younger and healthier. Your body does this so that you will look more fertile to males as your ideal time for mating and conception approaches.
So being tan isn't EXACTLY the deciding factor, it's more about looking glowing and healthy. Too much tan is actually a turn off because on white girls it makes them look like aliens and it screams high-maintenance. Try making your cheeks more rosy and a bit more shiny.
Most Helpful Girl
Because society has changed; it's actually linked to status and wealth.
Back in the day, when everyone was essentially working outside, on farms and such, being pale was considered really attractive because it signalled status and wealth -- it hinted that you could afford spending lots of time indoors so you wouldn't get a tan.
Then later on, after the industrial revolution, when indoors jobs became more common, it shifted -- now having a tan was a sign of wealth and status because it hinted that you could afford taking time off work and spend enough of it outside in order to get a tan.
Mind you, these are generalisations. Individual tastes can, and have, varied throughout the ages.