Or, why is QFT "better" than QM?
There may be many answers.
For one example of an answer to a parallel question, GR is better than Newtonian gravity (NG) because it gets the perihelion advance of Mercury right.
You could also say that GR predicts a better black hole than NG, but that's a harder sale.
For QFT versus QM, I've heard of the Lamb shift, but what else makes QFT superior?