Caps take up more space for a given text string in the same font size, but give better legibility for the same text area –you can read the same font size from farther away, or the other way around, you can use a significantly smaller font size with caps.
As far as I know, annotations in technical drawings have always been caps. For architectural construction drawing sets the cap font size in the full-size drawing meant to be discussed across a table or something will give an acceptablly legible 50% set for individual consumption only (typically ~2,5 mm for dimensions, although some people use 2,2; ~3 mm for text, at full size; not fun trying to read that at 50% with lowercase).
Also the supposed better flow in reading lowercase is irrelevant for the short texts used in architectural sets.
I personally dismiss construction drawings sets with lowercase annotations as the work of noobs (internally; I only bring this up with students or people working under my guidance) –draftsmen using computers before learning what a construction drawings set is.
Preliminary design presentations etc. are a different story.
PS: in text legibility, height is key (which is why road signs and license plates often use some narrow font); I think there are advantages to using Arial Narrow for dimensions, which I find more of a real estate problem than text, once they start cluttering the drawing with wall thicknesses and such.
