Because you are trying to use wired and wireless (on the same subnet) at the same time. Since your PC is on powerline, I assume your WIFI doesn't reach you and your printer. To continue to print wirelessly you would have to locate the printer so that your wireless router can handle the traffic.
If you don't want to do that, you will need to manually setup your wireless adapter with a different subnet than the main router and set a static IP address with NO GATEWAY. You will then have to re-IP your printer for that same subnet with a static IP address. No other devices connected to the WIFI router will be able to get to the printer with this configuration.