The Ruger originally only was available in 38 Spl, but would easily handle +P's.
The second incarnation is when Ruger started boring the chambers to allow 357Mags of 125gr weight to be used. Any heavier weights and the bullets would be too long for the cylinder.
The final version stretched the frame and added a longer cylinder to acommodate all 357 loadings.
As to the reason it is still cataloged? A very few areas restrict calibers of certain weapons. The city of St Louis for example allows security guards ONLY 38 spl. Possibly tradition and size might factor in also. While it rolls with the mainstream desires mostly, Ruger HAS made some interesting production choices over the years.