No honestly it isn't a tradition...I'm not certain why it really came about, I would assume because leverguns are really good with straight wall cartridges, and NEED flat nosed ammo, which is more typical of handgun bullets...

As far as history goes, I believe the 1894 Marlin, 92 Winchester and 94 winchester, following the 73 and 66 and Henry, and of course the spencer were available in pistol cartridges...but these weren't nearly as common as their "rifle cartridge" counterparts.

The reason they're so proliferent now because of their popularity in Cowboy action shooting, where having all three guns (two revolvers and a rifle) in the same cartridge is desired (although MANY shooters load different loads for their guns, one load for the revolvers and one for the rifle).

I fell in love with .44mag because of a Marlin 1894 "rancher", now I've got three of them, and four handguns in .44mag (not to mention five other leverguns and 7 other revolvers included in my broad battery).