- The degree symbol isn't part of ASCII, so it's not on the keyboard. Keyboards only did ASCII in the earlier years.
- When it became part of the upper-128 alphabet on the PC, laser printers took font cartridges, costing about $100 a pop.
Not many could afford multiple cartridges. You needed a math cartridge back then, so only one dept. even considered buying that cartridge.
You also needed a PC. If you took the document to another PC and printed w/o the math cartridge, you'd get something that didn't look like a degree symbol. Or, nothing.
- The guys really needing to author math used TeX; later LATeX, which is a meta language that typesets math symbols. It would take a lot of work to learn TeX just to get a degree symbol to print. They used mainframes. To get it, you'd have in your document: \usepackage{gensymb}
It just got left out. Too much work to look up. Too expensive to print out. Too much labor to use a typesetting language. People either just didn't use it, used an asterik in its place, or left a blank space and wrote it in using a pen after the print.
By the time everyone accepted workarounds, Macs and postscript (both expensive) laser printers came about, and people in that camp would print degree symbols. In fact, they spent a lot of time printing all sorts of fancy things that PCs couldn't.
It was probably another decade or more before the price came down enough that most people had the capability to print the degree symbol.
Windows and WYSWIG word processors on everyone's desks, and the CPU did image processing to postscript, but postscript was never popular in the PC world.