And it wasn't even a real failure; they contractually have to provide something like a 1 in 200 chance of failure or better, and in the state the vehicle is in they haven't or can't prove that they're meeting that safety margin, so NASA is choosing to go with an option that does have that safety margin. That's it. If they were to come down in it anyways there's still likely a 1% or less chance of failure.
