Which was influenced by tab stops on typewriters... which were adjustable, but were generally set at 1/2", 3/4", or 1"... which was the equivalent to 5-12 characters, depending on your font (monospaced, but sometimes 10 cpi, sometimes 12 cpi).
The VAX/VMS terminals we used at university to learn Pascal defaulted to 8 space tabs, on a 80 column screen. That certainly pushed some people towards using spaces.
I use 8-sized tabs with 72 columns in C. The human eye doesn't like moving all that much while reading and more windows fit one the screen even if it is annotated with git blame. In addition there is this argument from Torvalds, that you should mind your maximum nesting in a single function, which I also find useful.
