I wonder if the convention of labeling points in geometric diagrams with uppercase symbols ultimately derives from Greek mathematics, which was originally written in "majuscule" (uppercase) Greek script (in contrast to the "minuscule" script that was introduced much later (9th century?)). Certainly Euclid and Archimedes used only uppercase, and all of Descartes diagrams in La Geometrie (1637) follow the same convention.
It seems that middle- and high-school textbooks continue to use uppercase labels (is this only in the U.S.?), but college texts do not follow this as rigidly. This was brought home to me when I wrote a chapter for high-school teachers and the editors changed all my lowercase vertex labels to uppercase. I much prefer lowercase for point labels, although I do not quite know why I have this preference. (Maybe because uppercase seems like SHOUTING?) But when writing for an audience accustomed to a particular convention, it seems prudent to follow that convention.
My questions are: (1) Is the Greek majuscule script the origin of the uppercase diagram-labeling convention? (2) In so far as I am correct that the uppercase convention is followed up to high school but dissolves at more advanced levels, why does it persist to one level but dissolve beyond?