Ruger actually made a single shot pistol, the XP, I believe it was called. It just never made it pst prototype stages. it was pretty much a break open/tip up action w/ a Single Six frame.
With such a small market single shot pistols (excluding T/C's) it wouldn't be worth while financially to design/tool and test. There's other manufacturers that specialize in this type of handgun; leave it to them. besides, Ruger has so much on thier plate now, they cannot keep up with demand.
I'd like to see one myself though, just no hope for it.