The simple answer is: because of an anatomical predisposition. The short urethra, the thinning and dryness from lack of estrogen protection, and fecal bacterial adherence all play a role. In severely affected women, estrogen cream to the vulva (despite increased risk of malignancy) is worthwhile. I have also advised local application of diluted proviodine and antimicrobials (i.e. 50 mg nitrofurantoin) prior to sexual activity, with happy outcomes in many cases.