Sparkly, good question. The answer is because the Gemara and Shulchan Aruch say that it’s assur.
Rav Frand explains one possible reason behind it: A man is not likely to stare at a woman if he thinks she will notice, but if he is behind her, she won’t notice.
L’maaseh, he says that the Poskim say that today the halachos are different because there are so many women on the streets today. He says that on a date, a guy should let the girl go first. I wasn’t clear on what the halacha is if you’re just walking down the street or getting on a bus and you’re not a date.
Personally, I try to be careful when I’m walking down the street or getting on a bus not to walk in front of men, but that may be a chumra on my part (and obviously, it’s not even always possible).
It probably is a good idea though to try to be careful when getting on a bus, because unless your skirt is super-long, it may go up when climbing the stairs.