Sula is the main character of this story, the one who connects all the other characters to each other and the one around whom most of the action is centered. So it makes sense that her name is also the title. And it turns out that her name has several meanings that are pretty appropriate for her character, including "peace" (which is her last name) and "the sun," which kind of makes sense since the earth orbits around the sun, just as so many people orbit around Sula in the novel.
We can ask ourselves a lot of questions about why it's important that Sula gets top billing, though. After all, Nel is pretty central to the story, and some would even argue that she is the protagonist, so why isn't the novel named after her? And since Sula succeeds in making so many people mad, what does the title tell us about who might be the "good" guy and who might be the "bad" guy? At various times we might sympathize with Sula, really like her, or really despise her, so maybe it's this complexity that earns her the title role.