Not allowed to fly cryogens on the shuttle as a safety feature.
You also aren't allowed to carry propellant or use propellant on serviceable satellites, that was one of the design problems with the HST + shuttle.
It's not a shuttle specific thing it's the problem of anything man rated - you can't put anything dangerous onboard.
It's not a shuttle specific thing it's the problem of anything man rated - you can't put anything dangerous onboard.