The serial interface is the most simple way to connect the diagnosis interface of an automotive ECU to
a diagnosis PC.
There is no special hardware or software needed to program the RS232. So, a diagnosis software that communicates via RS232 could be run on nearly every computer.
If there is more performance needed (eg for CAN communication) there are also solutions for the
parallel port available.
The most powerful solution, of course, is an additional hardware, eg cards for ISA, PCI or PCMCIA.