Nubius hit it bang on basically, the 940 was originally, and still is, a socket intended for server usage. Since servers are some of the most demanding machines you can build, and dual cores were initiallu developed for server use, they have only released them on socket 940, for Opteron use and early FX processor use.
Incase you haven't realized, Opterons, especially in a dual core configuration, are still quite expensive and not practical for consumers, so a socket 939 dual core option hasn't been released yet.
However, AMD is planning on using socket 939 for dual cores, which will be released within less than two years according to estimates, and will fully support the Athlon 64 and Athlon 64 FX socket 939 processors.
I'm not sure what the plans are for mobile processors or the mobile Athlon 64s, I would think it would be more logical to migrate them onto socket 939 and completely eliminate socket 754 for good. There aren't very many released now anyways so it's hard to say.
And just for those curious about a dual core motherboard...