I wish Smith & Wesson could change the lookwork up slightly to have the cylinder stop offset like on the Colt DA in order to fit 6 rounds of .38 special in a smaller gun than the K-frame.
That might require more then a "slight change," and they would have to inventory 2 different small frames. Right now I suspect that 5-shot/.38 and .357 Magnum sales are strong enough "as is," and an additional 6-shot version wouldn't add substantially to it.
From a manufacturer's point of view the market is rapidly shifting toward polymer frame/striker fired/compact and sub-compact pistols. They may not be everyone's cup of tea, but they are more profitable for the manufacturer then more-expensive-to-make revolvers.
In the highly unlikely event that sales of their small frame/5-shot snubbies should slacken they might reconsider the 6-shot option, but I doubt it.
Historically, S&W .38 pocket revolvers' have been 5-shooters, going well back into the 1800's.