Valve deal with maps in a very different way than how they deal with models. It's very easy for Valve to see a model, decide if they like it and then incorporate it. Maps are a different story. They wont touch a map unless it is popular within the overall TF2 community. A workshop would be redundant and a lot of effort on their part tech wise, engineers, servers, software, etc.
For this reason i doubt Valve feel they even need a work shop as they can just use some server stats, have one of their employees do a little research for a few hours to find the map and play it on some populated servers and then find the author if they're interested.
A Boojum Snark: Don't listen to grazr he is probably drunk