WSL fuser command not working

I quote from the article Initial Evaluation of the Windows Subsystem for Linux:

Really, there’s almost no device emulation at all and /proc is pretty empty. You won’t see hard drives or removable media under /dev, nor will you see USB devices like webcams and joysticks. A lot of the useful things you might do on a Linux system aren’t available under WSL.

The answer is that /proc is an interface that needs to be implemented by Microsoft, since it needs to be implemented using Windows API calls. Evidently, Microsoft hasn't gone yet to the effort of implementing all these virtual folders, and it's unknown if this will ever be done.

Until Microsoft does this development, all utilities that use unimplemented interfaces will fail.

You will need to find alternative utilities that only use implemented interfaces, or access yourself the files containing the source data.