Why doesn't Atticus focus on Mayella's mental capacity and the possible molestation by Bob Ewell in To Kill a Mockingbird? During the trial, it...

This is an excellent question! First, there was no modern
rape kit available duing the 1930s and no medical evidence was taken at the scene.
Today, this would work in Tom Robinson's favor; no proof, no witnesses, and only
Mayella's conflicting statements. However, at the time of the story, the most important
factor was a simple one: A black man's word was NEVER taken over that of a white
person--even if they were a Ewell. Atticus did not attack Mayella's mental state for
several reasons: For one, she was probably the brightest of the Ewell family, and it was
not unusual to have not received a proper education. (Remember, Atticus never went to
school himself.) More importantly, Atticus would probably have turned the jury
completely against him had he suggested that Mayella was mentally unfit. He had no
medical proof, and it would have been socially unacceptable to treat a woman (even
Mayella) in such an ungentlemanly manner. As for the possible molestation by Bob (and
there are undercurrents of this possibility in the story), neither of the Ewells would
have admitted the truth even if it had occurred. Mayella feared her father, and this
secret would probably have been confined within the
family.

Obviously, a case such as this in the 21st century
would have probably never gone to court. The extreme racial bias would not have been
evident--by either the prosecutor or the jury--and some sort of medical exam would have
been necessary. Even if Tom had been brought to trial, which was unlikely, a conviction
would probably have been overturned on appeal (as Atticus hoped it would). The Ewells'
reputation would have worked against them, whereas Tom's relatively clean background
would have trumped the word of the Ewells.