yes there is a reason. it's called geometry. women's bodies, as mentioned just above this post, have different proportions. therefore the bike's require different proportions as well.
even with that, a good bike fitting is important if you are going to ride a lot. a bike fitting will take an hour with a trained professional and will make more fine-tuned adjustments on the perfect seat height, distance from seat to handlebars, handlebar adjustments, etc.... those measurements are then saved and used on a future bike purchase. i worked at a bike shop, we had guys buying 10,000K bikes - take their measurements from their last whip and apply them there, and then still fine tune it.
the bike fitter puts them on a bike on a trainer and works with them until it's perfect to the 1/10th of a mm.