Because women are financially more independent and there are almost no pubs with men-only mentality; wine bars/cocktail places target women and there's a huge choice of booze that tastes sweeter than real ale or whisky.
It's perfectly socially acceptable for women to drink. It's a badge of honour to be lashed most weekends for many.
It's more acceptable and safer than illegal drugs, and eating your body weight in comfort foods.
It's seen, if it's in a nicely-shaped chilled glass and there's an olive or two nearby, as sophisticated and normal, ooh, continental even.
Cocktails make you feel liberated and SATC-ish.
Your mates all do and there's nothing more bonding than telling someone you love then at 3am, while opening another cheeky bottle.
It's become a huge part of the UK female experience.
I can't say it's hugely enlightening, attractive, sophisticated, or clever.
It's a physical time bomb, getting wankered on a regular basis, as well as making you spectacularly dull company.
Those I know (myself included in a previous life) who drink the most and get very aggressively defensive about it are usually the ones who feel sad and empty and drink heavily to make up for life's disappointments.
But saying so out loud is social suicide.