There's a lot of social pressure for women to wear makeup which is especially hard to pull off if you bike to work. A woman with tasteful makeup is perceived as more professional, even if it's just subconsciously. I'm sure there are men here who will object to that statement, but 9 times out of 10, when a guy thinks a woman is wearing "no makeup," she's actually wearing primer, foundation, concealer, mascara, eyebrow pencil, lipstick, and blush and just hasn't done anything too obvious.
Because of this illusion, women don't want to be seen bare-faced by their coworkers (because society's expectation of what a woman looks like without makeup is actually a woman wearing a lot of makeup so the real no-makeup look is shocking). Now a woman who wants to bike to work has to decide if she'll hope her makeup stays looking good after a bike ride, do her makeup twice, or go without makeup for the bike ride (and risk being seen without makeup going into the office). This doesn't bother all women, but it's no wonder that a lot of us choose not to go through this minefield of issues.