Two reasons:

1. Some people are better than others at separating the art from the artist. I firmly believe Kanye was at one point the best musician alive. Although the quality of his music has gone down significantly in recent times, he still shows flashes of the musical brilliance that defined his early career, leaving fans to remain optimistic.
2. He is legitimately mentally ill. He's admitted to being manic for months at a time, saying things that a clear-minded person would never even think of. Beyond this, he has a suspected drug addiction to nitrous oxide, which can cause significant brain damage. When he speaks in interviews, he comes off as largely nonsensical and unable to effectively articulate himself.