You can, but the point of subpixel rendering is increasing effective resolution, and for English text, horizontal resolution is more useful than vertical resolution. See: https://www.grc.com/ctwhat.htm.
It's not just English text, it's virtually every written language. There are very few that use a vertical orientation, and fewer still that use only a vertical orientation.
Hebrew, Georgian and Arabic all benefit equally from sub-pixel resolution. Even vertical Chinese would be improved by having more detail on each character.
