Why can't Klaus make hybrids?
Matt eventually contacted his dead sister, Vicki, who told him that the fact that Elena was still alive was the reason why Klaus couldn't make more hybrids. Can Klaus make hybrids without Elena's blood? Now Elena is the missing key to create a hybrid, that's why Klaus needed her blood. Katherine's blood can't be used, as she is a vampire now. That curse required to sacrifice one vampire, one werewolf and one Doppelgänger.