We know from earlier chapters that the Ewells are poor, and according to Atticus. they "live like animals". The Ewell children only attend school the first day of the year, their taken care of by their older sister, and the town allows Mr. Ewell to break laws others must adhere to. If they didn't, the children might starve.
Tom Robinson, on the other hand, is a married, hard working, church going, family man.
The townspeople take Ewell's word above Robinson's for no other reason than because Tom is black.
Please ask your questions separately.