Because they've had to delay the Antares resupply launch until after this is done, and if the other coolant loop should experience a problem and shut down, then it's time to evacuate the station. Not a good day. The problem is manageable right now, but it leaves them with no redundancy in case of another coolant problem. It's a rather complicated job to fix it too, requiring multiple spacewalks, so this isn't something you can just do on a whim if further problems start to develop. In the best traditions of NASA fashion, they have come up with a simple but innovative series of work-arounds to keep Hopkins safe in case the water system in his suit leaks again.