Why do most probiotic supplements fail to deliver?

A probiotic without prebiotics is not an effective gut supplement. It's an expensive one way trip. Probiotics are live bacteria - and live bacteria need food to survive.

That food is prebiotic fibre. Without it, the bacteria arrive, find nothing to feed on and pass straight through. They can't colonise. They can't have an effect. Most probiotic supplements contain no prebiotics at all. The bacteria never stand a chance.