why the PWS needs the transponder to work?
To answer the "why" part...
If the transponder is in the ON/AUTO etc. position there is a good chance you are about to take-off, and not sitting in the hangar with the PWS accidentally switched to AUTO. Stops it transmitting without good reason.
when we have an engine failure the STATUS says both PFD on the same FAC, why
Nothing to do with an engine failure....