In "To Kill a Mockingbird" why do you think people decided that Boo Radley killed Bob Ewell and that Bob Ewell didn't just fall on the knife?I...

People conclude that Boo Radley killed Bob Ewell because the evidence seems to point in that direction.  If you look really closely at the evidence and facts that we DO have (even if we don't have a clear statement of guilt), all of it points logically to Boo Radley.  Look in chapter 28, at the actual scuffle that occurred between Bob and the children.  It is written through Scout's perspective, and she was blinded by her costume, but through sounds, we know that the man squeezing Scout to death was "suddenly jerked backwards."  Scout thinks Jem had done that, but we learn later that Jem's arm was too badly broken to have pulled anyone away.  Someone else must have been on the scene, which, we conclude later, was Boo.  So Boo jerks Bob off of Scout, and then she hears "scuffling noises" and a man wheezing as if out of breath.  He

"leaned heavily against [the tree].  He coughed violently...[then] began moving around, as if searching for something.  I hard him groan and pull something heavy along the ground."

After this, Scout hears that same man "walking heavily and unsteadily toward the road." Then, Scout feels around and feels a man on the ground, a belt buckle, and a face.

So, to interpret all of that information, using inference and what we know of Boo, Boo pulled Bob off of Scout, they fought, and then there was silence except for Boo's heavy breathing.  Boo was out of shape, used to being indoors, and later on, coughs a lot.  So, it must have been Boo coughing and trying to catch his breath.  Bob was silent. Then, we hear Boo dragging what must be Jem towards him to pick him up, then walking "heavily" away, which means he had Jem in his arms.  Then, Scout feels what must have been Bob on the ground--and his is face up.  That is key, because they later concluded (in order to protect both Boo and Jem from accusations of murder), that Bob tripped onto his knife. If he had done that, Bob would have been face down on the ground, not face up.  So, someone must have stabbed him.

So, you're right--the novel never specifically says that Boo stabbed Bob, but if you look very closely at the details in chapter 28, and put them together with what we know of Boo and what comes afterwards, then the logical conclusion is that Boo did indeed kill Bob.  I hope that those thoughts and explanations cleared things up for you a bit; good luck!