below...
Could it be because they all started with current loop tty interfaces?
Most of the old DEC guys started with teletypes.
Having struggled with a breakout box and different mini and micro vendors implementations of serial ports... Ugh. And in three-wire the use of Xon-Xoff varied big time. No standard was the standard. IIRC the IBM Series/1 had a different 9pin layout than the PC/AR. Why?
RS-232A/B/C was DB-25 P for the DTE (terminating equiment - a.k.a. terminal) and S for DCE (communications equipment - a.k.a. modem). It was standardized. At one time, I (sadly) could quote the paragraph number....
Then in 1978 #$%^& Lear Seglier put a DB-25S on a DTE (terminal). They were the cheap terminal vendor and all hell broke loose.
The PC/AT used 9 pin because the back of the unit was small and -- well there could because IBM said so .... But at least the IBM engineers kept to Plug and Sockets from the standard. I did not know the Series/1 used 9 pin. Learn something new.
At least DEC was reasonably consistent until they moved too the Vax modified RJ design.
Indeed - that was a huge issue - the modified RJ block -- sigh...