It's because on a terminal, CR moved the cursor to the left column and LF moved the cursor down one line. That was also true for dot matrix printers, and paper-based terminals like a modded IBM Selectric or the DecWriter.

So if you wanted your file to display properly when type'd to the console or pip'd to a printer, you needed CR,LF make it work. LF alone would give you text in a barber pole pattern, and CR alone would give you the last line of a file on a terminal, or a horrible mess of over strike text on a printer.

Ah, I remember those days. If someone made the mistake of just using CR alone in a big file they spooled to the lineprinter, it sometimes quite literally would halt and catch fire. There was a fire-extinguisher placed next to it for a reason.