I think it may partly be a matter of convenience.
As most of you know, you cannot buy drop in tapered rollers, they are taking existing sets, and modifying OD or ID.
If you are making your own trees & stem, or just a stem, you have choices, of other sizes, ie a smaller OD race will fit easily with a shim bushing.. say a 22mm or 7/8 ID.
So it might be a case of what was easy, and yes, I think taper on the lower is a help. But the convenience of quick tree or complete fork removal with 2 tapereds, is the icing on the cake..