Because most drugs are metabolized in the digestive system and waste by-products need to be cleared out
of the system by the kidneys. Most
of the by-products are toxic, so they have to be removed from the body - and the already damaged kidneys can find this work hard to perform, so some toxins end up circulating back in the bloodstream.
Benazepril, thankfully, is metabolized chiefly by the liver. So this is actually a better drug than many for this type of treatment, as it spares the kidneys.