Below is an excerpt from "100 Years of Martin's Landmark Ukuleles". It indicates Martin probably started building the SO sometime in 1998 or so. I can shed a bit of light as to why they stopped making them. The first production models of this late 1990's release didn't quite live up to Martin's legacy standards and just appeared to be cheaply made with a pretty big price for what it was. Also, no position dots on the side or fretboard as well as no kerfing. Some complained it only had 12 frets, but to me I prefer a 12 fret neck.
"In 1997, just three years after removing ukuleles from production, Martin began building “Backpacker” model ukuleles at its factory in Navahoa, Mexico. Soon it was making a standard-shaped soprano mahogany model at the same facility, a model called the S0 ukulele. As interest in ukuleles continued to grow, Martin decided the time was right to begin building ukuleles in Nazareth again. In 2006, Martin re-entered the high-end ukulele market by re-issuing the Style 5K ukulele, a model that had not been made in regular production since 1938. The next year it debuted the Style 5 “Daisy” ukulele, a curly mahogany instrument modeled after a ukulele that C.F. Martin III had built for his wife in the 1930s.
More affordable models soon followed, some made in Nazareth and some in Mexico. In 2008, Martin introduced three new Style 3 models, in mahogany, koa, and cherry. In 2010, they added a more affordable 0XK ukulele, made from a high-pressure laminate. In 2011, the new line of Style 2 ukuleles debuted. Besides the soprano, these included a concert and tenor ukulele—Martin’s first larger-bodied ukuleles to be re-issued. The next year, they added C1K and T1K concert and tenor ukuleles made in the Mexican facility."