Why is one of my USB ports not working?

Usb ports do have power lines but also data lines. Since you tried the available hardware resets it seems to be you have a failure along the data rails. If it's your right port it might be sufficient to replace the I/O board it belongs to. If it's the left port, that sits right on the logic board itself and needs component level troubleshooting.

The why just has one simple answer..because things break unexpectedly, that happens with cars, satellites, space shuttles and Macbooks too.