[emodendroket]

The problem is they don't look the same. So imagine, for instance, Я instead of "R" or И instead of "N" (I don't think the sounds are actually equivalent but let's run with it for the sake of example). Not insurmountable. One could still read a text with these substitutions. But it'd be distracting, and extra detrimental for people who don't speak English as their first language.

[digisign]

The ones in the center are in all three sets, they do look the same. The outer areas are out of bounds.