It's because you didn't put the top case on from the back to forwards.
Those clips have to fully seat before you press on the optical drive side (front) of the case.
To fix the problem; carefully remove the case again and inspect that you didn't pinch or sever any antenna cables. Then with the Mini upside down, seat the base (rear port side first) into the case and seat it carefully. It will be tight, so keep pressure on the rear port side and have your putty knife handy to guide the side clips back in.
Take your time so that you don't damage your Mini -- a sloppy job can be spotted a mile away if you try to get future service at an Apple Authorized Service Provider or Genius Bar.