I'll take a swing at this
I believe being hung on the cross caused fluid build up in the lung, and problems of various kinds. Some of these problems could be lessened by pushing up with the legs/feet/ankles.
I'm gonna guess that not having the legs to use (as in they are broken) would prevent the relief they provided, and result in a much quicker death.
Now I'll step back and see what the smart folks say about it
Here's a
link to a description of what happens during a crucification. I'm not saying it's accurate, but it seems to agree with the little I've read about the subject. This is an attempt to describe what Jesus would have gone through, and is from what I assume to be a Christian source. Was the first link Google gave that described what happens physically.
"At this point, another phenomenon occurs. As the arms fatigue, great
waves of cramps sweep over the muscles, knotting them in deep, relentless,
throbbing pain. With these cramps comes the inability to push Himself
upward. Hanging by His arms, the pectoral muscles are paralyzed and the
intercostal muscles are unable to act. Air can be drawn into the lungs,
but cannot be exhaled. Jesus fights to raise Himself in order to get even
one short breath. Finally carbon dioxide builds up in the lungs and in
the blood stream and the cramps partially subside. Spasmodically, He is
able to push Himself upward to exhale and bring in the life-giving oxygen.
It was undoubtedly during these periods that He uttered the seven short
sentences which are recorded:"