I think it is what the uniform represents. Most men in uniform have to conform to a predefined set of standards. The standards that these men must now embrace range from self maintenance, call to duty, development of pride in the organization, being ready to assist and help other, self reliance, and courtesy to those they meet to name a few.
These and other traits of the standards that these men have embraced in wearing the uniform, fall under a trait that many woman seem to specifically look for in a man. And that is the ability to commit to a relationship!