There's no need for taipan venom to be as strong as it is just to kill a little long-haired rat.

That's a consequence of its attack style, it doesn't just strike the prey and then retreat, it actively subdues it and doesn't let go.

So there's an evolutionary pressure to make venom as fast acting as possible so that the prey has fewer chances to injure the snake before dying.

Black mamba in Africa is in a similar situation. It's an arboreal snake, so its toxic venom helps it to hold on to the prey.