why did sheriff tate wanted poeple to think ewell fell on his own knife?

The truth about Boo Radley would have harmed Boo himself. Boo could not handle the attention and it was better for all involved that Bob "fell on his knife".

the sheriff insists continually that Mr. Ewell fell onto his knife and killed himself, which irritates Atticus, who wants Jem to be treated as fairly as anyone else and not have exceptions made. After much arguing, finally the sheriff yells out that he's not trying to protect Jem (he is trying to protect Boo). The sheriff urges Atticus, this once, to accept the situation even if it's not perfect according to law: Mr. Ewell was responsible for Tom's death, and the sheriff urges Atticus to "let the dead bury the dead." He says that it would be a sin to drag shy Boo Radley out into the limelight, and declares officially that Mr. Ewell fell on his own knife. Atticus, deeply moved by this revelation, asks Scout if she understands. Scout assures him that she does, explaining that having it another way would be like shooting a mockingbird. Atticus looks at Scout with a sense of wonder, and thanks Boo for the lives of his children.