If you knock someone "out", they don't immediately get back up.
That was a combination of pride and HEART that allowed Ali to stand up so quickly. It was the first time he'd ever been knocked down, and he didn't want to give Joe one extra second of seeing him trying to recover on the canvas from that blow.
R.I.P.
Smokin' Joe was one hell of a fighter, in the days when HW boxing was at it's best.