Because art is not inherently successful, and using commercial success as a metric on whether it's "good" is philistinism. Art that is "bad" is at the very least interesting in more than one dimension. I have come to appreciate art that to most would be off-putting because where it excels and interests me, it does so in such unique ways I would never expect. There's no telling where those seeds of ingenuity will grow to and who they will inspire.
We need "bad" art as much as we need "good" art.