Why would a patient in renal failure get decreased cardiac output?

They wouldn't: But the reverse is true. Someone with low cardiac output underperfuses the kidneys and commonly gets "pre-renal azotemia" which is a form of poor kidney function. People with advanced renal failure are commonly volume overloaded which is often mistakenly called "heart failure" (really it's circulation failure). Is that what you mean?

Complex: Even though the mechanisms are complex, one way to understand is that if you lose the ability to filter your blood and eliminate extra fluid from your body (one of the kidney's main functions) your heart will be burdened with pumping a lot more blood than it was intended to do and will therefore fatigue and not pump as well (same as any other muscle would if forced to work "overtime").