The simplest explanation is likely the best, even if its validity isn’t always intuitively obvious in advance.

USB took over, but motherboard manufacturers retained the 9-pin serial header for legacy compatibility. You may not know about it or need it for any of your devices, but it still has real-world uses ...

The parallel port is gone as of the time I'm writing this. But its demise was long and drawn-out, even as supposedly better alternatives came along and tried to take its place. He ...