In all honesty, it's karlovic's fault. If he withdrew around the same time nadal and monfils, I think haas would've made the trip to London.
Edit: was haas even on the alternate list?
no because the German committee didn't ask him to enter
His only chance to come would have been that the German committee asked him top come : he would probably have received an invitation from the ITF (not sure because the ITF gives priority to players who have been highly concerned with the Davis cup)
... or if he had been entered in doubles