forget 26" and call it "standard guitar scale length" if that makes it easier. Deans work better than the shorter models marketed under other brands.

I wouldn't doubt that guitar banjos have been around a long time but that doesn't mean that they were an improvement over banjos with fewer strings.

I believe that the tenor banjo was invented in 1915, and there's nothing better for cutting through brass instruments. The instrument was used very widely in jazz bands until the depression of 1929. Ok Django played a guitar banjo for a while in the late 20's, but most bajoists were using the tenor (and most of the rest the plectrum banjo) by the time the dixieland bubble burst.

I'd guess that guitar banjos emerged at the beginning of the jazz age for the same reason that tenor guitars emerged when dixieland died and guitar-accompanied swing came in - the "parent" instrument went out of fashion and professionals needed to make the newly fashionable sound quickly if they were to stay in work. Moving the stringing pattern to a different body saved learing time.

That's my rough and very generalised theory anyway. Enjoy your guitar banjo (or is that banjo guitar?)