As I understand it, if you want the blood to remain viable you have to get it on ice immediately or it starts to congeal. The timeframe is short. I don't know how short but you can look it up. If the heart stops beating there is the problem of getting the blood out, which is much easier when you have the heart pumping it into the area you are extracting it from.
There is also the problem of no oxygen and a build up of CO2 and toxins that are not being removed through the natural processes. The chances of success are much higher if he's alive.
On the other 72 superhumans, there was no guarantee any of them had the regenerative properties of Khan.
Thawing them out must be a lengthy process. I'm sure if Khan had died and Bones wasn't able to get to him soon enough to extract use-able blood, he would have then turned to the others, but why risk it?