FWIW:
I can tell you why I THINK the SW99 never sold better....
Same reason as why the excellent P99 didn't sell better...
POOR MARKETING...
Also, the confusion as to "who" made the P99, and why the SW99 was even made when it was basically a P99 excluding the slide and barrel, and the price was sorta high, and POOR MARKETING, etc..
For example if you read the various firearm forums about the P99/SW99 you will find out that many folks think because the P99 has "S&W" on the barrel it's made by S&W.. Nope, just imported.. Some folks think that since the bottom end of the SW99 say's Walther it's made by, uh... well, it's confusing to say the least. I don't think this Walther licensing the manufacture of the Walther PPK is clearly understood by most firearm buyers, and the relationship between the P99 and SW99 is so close to incest that nobody can figure it out.
I do know that Walther fans consider the P99 superior to the SW99 because it's "all German" (so was the Tiger tank for what good that did them in WW2), and the barrel and slide of the SW99's made in the good old U.S. by S&W were somehow inferior.
(The all American S&W PPK's get the same rap vs the older German PPK's.)
I do know that my buddy bought a new SW99 because he couldn't find a P99 at the time, and for his trouble he saved money, got basically the same pistol anyway, got a lifetime warranty vs the one year Walther warranty, and his SW99 shoots and functions just as well as my P99.. We have shot them "head to head" and I certainly couldn't tell any difference between the two, and you just know I was looking for some...
My buddy loves his SW99, and I think they are excellent firearms.
Just personal opinion, no offense.
Jesse