The Orbalisks pumped adrenaline and intended to make him mindlessly rage:

He focused his power and channeled it through the orbalisks, letting them gorge themselves on the dark side. In return they pumped a fresh dose of adrenaline and hormones into his blood, allowing him to generate even more power in a cycle he repeated over and over until the moment before impact.

Bane snarled in anger at her betrayal, his rising anger fueled by the chemicals the orbalisks were pumping into his system.

There were other considerations, as well. Twice now he had lost himself in a bloodrage, thought and reason replaced by the mindless urge to destroy anything and anyone in range. The first time it happened he had left their camp in ruins: a foolish and pointless waste of resources.

If the orbalisks drove him into another rage, he might kill her. 

Normally they fed off the dark side energies that naturally flowed through him, but the creation of the Holocron demanded that he channel all his power directly into his work. The parasites were slowly starving, and in response they were flooding his bloodstream with chemicals and hormones intended to drive him into a mindless fury so they could gorge themselves on the dark side as he unleashed his rage.

- Darth Bane: Rule of Two

A decade earlier he would have eagerly engaged them in physical combat, his body pumped full of adrenaline released by the orbalisks that had covered his flesh. Swept up in a mindless rage, he would have carved a bloody swath through their numbers, hacking and slashing at his helpless enemies while relying on the impenetrable shells of the orbalisks to protect him from their blows.
But the orbalisks were gone now. He was no longer invulnerable to physical attacks, yet he was also no longer a slave to the primal bloodlust that used to overwhelm him. Free from the parasitic infestation, he was able to dispatch his enemies using the Force rather than relying solely on brute strength.

- Darth Bane: Dynasty of Evil








Bane theorized he might actually be getting weaker due to the Orbalisks:

While it was true that he could channel his power through the creatures for a temporary increase in his abilities, over the long term they might actually be weakening him. They were constantly feeding on the dark side energies that flowed through his veins. Was it possible that, after a decade of infestation, his ability to draw upon the Force had been subtly diminished?

Bane still could not discount the theory that the orbalisks were responsible for his failure, bleeding him of the dark side energies he needed to draw on to complete the procedure.

- Darth Bane: Rule of Two