I think the reasons behind there being lots of attractive people-- and women-- in plastic surgery & derm are obvious. In addition to practitioners being naturally inclined towards aesthetics, there is a very real amount of pressure to look good. You unconsciously, and probably consciously, do not want some schleppy guy in charge of your looks.
As far as peds goes, well, if you're the sort of girl who really puts a lot of effort into your appearance you're more likely not going to be inclined towards a field where that is more or less impossible. In surgery you wear shapeless, manly scrubs. Your time to work out is significantly constrained. Your hair must be pulled back. Jewelry is impractical. If that bothers you, well, go into something where you can wear nice clothes and style your hair as you please and hit the gym a few times a week. Like peds.
And at risk of invoking the wrath of Khan, I think a desire to maintain your appearance is very much linked to a desire to attract the opposite sex. I think female surgery residents are more comfortable being partner-less than others (though the pain of heartbreak and loneliness is univeral). I get the vibe from the peds girls especially that babies are on their mind in a non-professional sense as well.