I’m not familiar with this particular treatment but drugs are only so effective. Once you hit a particular concentration you are no longer getting meds to the appropriate receptors since they are saturated. Additional medication will just cause side effects since increased concentrations of drugs will just start causing side effects by interacting on other receptors. Using a different medication allows you to get the same action but without the increased drug concentrations.

Thanks for the correction

As someone with a working knowledge of pharmacology and anesthesia, this right here.

Edited for terminology and corrections: The GABA receptor has a maximum effect that can be elicited from benzos. Once the receptors on it are bound by benzos there can be no further action, a ceiling effect. When other substances are added such as alcohol and narcotics they cause respiratory depression by means of other receptors, in the case of narcotics the opioid receptors, mu, kappa and delta. In the case of alcohol it ties up a different receptor on the same GABA neurotransmitter so it potentiates the effects caused by GABA activation. Other substances can also have the same effect as alcohol. This is essentially what happened to Anna Nicole Smith.