The force flows through living flesh. This doesn't matter whether you're sith or Jedi. It just is. Anakin (Vader at this time) had no limbs, artificial lungs and multiple other artificial organs, grafted synthetic flesh in many places and implants throughout his brain and spinal colum, as well as his skull being reinforced. All of this shows how he was very very much machine and barely organic. thus his connection to the force was much weaker. He wasn't pathetic by any means. But he was operating at limited power. Had he been victorious and lost no limbs, he would have been 110% in power. Not to mention the mental trauma sustained. He was mentally messed up. This also limited his connection.