Here's why:
If they put in GS and Pad support, people using them would be easy targets and thus cry about getting owned to BW and how this game is blah blah blah and just making the complainage last indefinitely.
Thus I am sure BW made a conscious decision to leave them out.
Mouse and keyboard is much more quick and accurate than a clunky joystick / pad in which you will no doubt come to realize once u put in time to get used to it.