My guess is that the .38 is marketed to certain places or applications where there is a legal or liability-related reason not to shoot anything called "Magnum."
Mexico, for example, doesn't allow you to keep anything bigger than .38 at home, AFAIK. Australia has a similar, though not identical, restriction that I don't quite understand (no gun laws make sense, really).
Some PDs may not want their officers to carry Magnum BUGs, for whatever reason.
Otherwise, I can't see why Ruger would bother making the .38 version at all, given what's been posted above already.