You have only specified the x and y sliders in the configuration file so when your sketch attempts to find a matching device it will stop at the first device that have sliders called X and Y, probably the mouse. 
There are two things you can do
- Include something unique from the joystick in the configuration, even though you don’t use it in the sketch.
- If you are using the latest version of GCP you can use the
filter(GCP.STICK)option see this guide