Agree with most of what was said in the above two posts. Mostly, it is just no longer cost - effective to make the 3rd Gen. pistols any longer.
Most Law Enforcement is going with some sort of stiker-fired polymer, and the civilian market is doing the same. Not very many people go for a metal framed DA/SA auto anymore.
Not to mention, the cost of making something along the lines of a 5906 today, I'm sure S&W would have to charge somewhere between $800 - 1000. People seem to be willing to spend that much on a 1911 type pistol, but I really don't see a market for a SA/DA pistol in that price range, when you can buy an M&P for almost half the price.

From what I've seen, most metal framed SIG's seem to go for around $800+, and they seem to be starting to get more aggresively into the polymer framed pistols. I don't know how many P220/226/229 pistols they sell nowadays, but I would not be surprised if SIG ceased making most of their metal framed model DA/SA guns in the next decade or so.