Because serial is a trivial interface to implement in home brew electronics, just throw in a MAX232.
Even many major consumer electronics are still developed via a serial console. Sure they have fancier connectors to talk to your PC and the network but serial is so simple it's practically idiot proof. So it breaks far less often, makes new hardware much simpler to bring up, and it allows you to debug the fancier interfaces without interfering with their operations.
