Lower-cased letters have both ascenders and descenders. Fitting both of those in 8 pixels is doable but leaves you with either the clipped ascenders or mangled

Lower-cased letters have both ascenders and descenders. Fitting both of those in 8 pixels is doable but leaves you with either the clipped ascenders or mangled descenders that you see in old 8-pixel lower-cased typefaces - your method works, but the results are poor. Upper cased Latin letters are far more amenable to rasterizing down to small sizes than lower cased. Though the real reason behind the question is that teletypes printed in upper case, early computers copied teletypes and followed that convention which carried on through the 8-bit era.