I believe that most women who don the hijab, niqab or burqa are either forced to by their men-folk, or they wear them out of some skewed quasi-religious/feminist ideology that compels them to make a political statement, of sorts, out of their Islamic garb: I am Muslim, Hear Me Roar. For others it's a litmus test for their supposed 'piety', as if you can't be a moral person without covering your head.
There are also those Muslims (and many of them female) who claim that women are obliged to dress modestly because they don't want to tempt men, who I suppose have absolutely no control over their sexual instincts if they are aroused by a woman's ponytail.
Islam commands women to cover their bodies so as not to trouble men who are weak and unable to resist temptation. In Islam, men and women are commanded to dress modestly and not appear "naked" in public, even in all male and female situations.By covering their bodies from head to toe these women feel they are not calling attention to themselves, and therefore saving men from committing fitnah by lusting after them. This might very well be true in Arab lands where the hijab/abayas etc are the norm, but in Western countries where it's more of an oddity than anything else, they stand out like sore thumbs, which would seem to defeat the purpose.
According to Islam, men are supposed to dress modestly as well, but unless they are wearing a Thobe (tunic) or shemagh (head dress) Muslim men are usually dressed in Western clothing (often jeans and t-shirts), and unless they are accompanied by their hijab or burqa-wearing wives, there's no way you would identify them as Muslim. One reminder of gender inequality in Islam, and its inherent hypocrisy.
Ironically, the resurgence of interest in the hijab came after 9/11, but not satisfied with the dull colors and styles that typify modest Islamic clothing, Muslim women who wear the hijab, niqab or burqa (for whatever reason) are now clamoring to prettify their outfits, and Islamic high-fashion has emerged.
In Australia, an exhibit - Faith, fashion, fusion: Muslim women's style in Australia- has just opened at the Powerhouse Museum in Sydney, which features the latest styles and trends for modest women's clothing. Some of the outfits are actually quite attractive, but many are as form-fitting simply longer versions of their Western counterparts. Designers have brought color and style to the normally drab garments, but fail to see the utter hypocrisy in trying to look fashionable while trying to maintain a semblance of so-called modesty. Now, not only are they drawing attention to themselves by wearing a veil, they are attracting even more attention by making themselves look trendy.
ABC Australia video of the 'faith, fashion, fusion' event.