Because your name is your identity. The one you’ve had your entire life, and one you may not be willing to alter simply because your status has changed from “single” to “married.” It effects who you are and who you understand yourself to be in a very fundamental way.
Because sometimes a woman is an only child, or part of a family with only female children, and she wants to ensure that her family name continues into another generation.
Because it subsumes your identity to that of your husband. For example, check out CrazyCatLady’s thread about a friend of hers who insists on addressing correspondence to “Mrs. Dr. CatLady”.
Because many women are professionals who are known by their given names, and to change that name could confuse their clients, associates, readership, what-have-you. To change your name in any profession means risking that many people will lose track of you entirely, think you’ve left the company, or (if they meet you after your name change), have no idea that your previous work was done under a different name. Awards you’ve won, articles you’ve published, anything at all that you have created won’t turn up in research about you under your new name.
Because it’s an absolute PAIN in the ass. Try filling out a bazillion forms at Social Security, the Department of Motor Vehicles, the bank, your credit cards, your insurance, your benefits, Payroll at work, getting your email address changed an letting everyone know, your frequent flyer and other loyalty accounts, and having to tote around your birth certificate, marriage license and old forms of ID with you to each one. And each one takes two hours at minimum, and those hours are always between 8 am and 5 pm when (surprise!) most of us have to work. Booking travel before your name changes and then trying to travel after the change is difficult.
From the tone of your OP, I’d bet that you would never consider changing your last name to your wife’s if you get married. Ask yourself: why not? Is there more to your unwillingness than simple tradition? And if tradition is your only reason, why is that a good one?
By the way, I am married and I did change my name. But I thought long and hard about it, and completely respect any woman’s decision about her name, whether it changes or not.