Charles Ellicott Commentary John 19:33

"but when they came to Jesus, and saw that he was dead already, they brake not his legs:" — John 19:33 (ASV)

And saw that he was dead already, … The only explanation of their not breaking the legs of Jesus seems to be that the purpose of the crurifragium was to ensure death, or, in any case, prevent the possibility of escape. Crucifixion itself would not necessarily cause death for several days, nor, indeed, at all; but Jesus had by His own will committed His spirit to His Father.