[ImprobableTruth]

Because we don't live in a world with UBI, so we have to decide how to allocate finite resources. The alternative to using some arbitrary standard of commercial success is some other arbitrary standard (unless you introduce UBI).

If we want to optimize for enrichment of people's lives through art, "how many people are willing to pay for it" seems like the best proxy we have. Not to mention that people with some financial success will be less costly to subsidize.

Like, if I had to choose between subsidizing two musicians projected to make half of living wage and a single musician earning nothing, to me that's a pretty obvious choice. The musician earning nothing might be a true visionary who will be recognized in the future, but the odds are far more likely that the music they produce just doesn't hold much appeal.

Now, commercial success is of course not a perfect measure for "enrichment", but, again, what's the alternative? Currently it's some government body deciding, though this holds the quite elitist assumption that the taste of the art establishment is better.