Most Helpful Girl
I think it is because woman's gown requires a lot of detail. Two girls may both wear the same size but their cups/waist/etc might be different. As a result, you have to get the dress custom tailored to fit your specific needs. So, opening a business to rent out wedding dress might not be profitable since most of the dresses would have to be altered so many times and get destroyed in the process. In addition, some women like to keep their gowns as memories, or maybe pass it down to their child(which I doubt it happens a lot).
Meanwhile, men also do have the option of purchasing a tux. However, tux is not as detailed and most men do not attend occasions enough where an owned tux is suffice. I'm not saying guys don't get it tailored but price difference is significant when it comes to rent vs. purchase. Hence, renting out a tux would probably make more economic sense than purchasing it.