I have a 2003 646 also, great gun.
My guess would be that the titanium cylinders are one big reason they haven't made any more. I don't think they ever caught on well with the buying public at large and S&W seems be getting away from them. The walls of a steel .40 cal L-frame cylinder would be too thin.
Plus when they were produced they didn't sell well, gun shops were practically giving them away. They may be gunshy about putting another one out there.