Probably availability.
There were about 11,000 HDs made prior to WW II, but fewer than half that number of Outdoorsmen -- only about 500 a year, on average.
It may have been either impossible or extremely difficult to find an Outdoorsman.
Let's face it, too.
King custom target sights are a lot nicer than the stock "field" style adjustable sights on an S&W Outdoorsman.