Why is it easier for members of our society to reach a consensus as to right and wrong in certain behaviors than on others?

Since you have placed this under law and tagged it with
criminology, I am assuming that you are asking about why we can agree that some things
should be crimes but cannot agree on others.  One reason for this is that some actions
have clear victims and others are less clear.

We can all
agree that one person's actions should not be able to clearly harm another person.  That
is why we can all agree that murder is wrong.  So why can't we agree on whethersomething
like abortion should be legal?  It is because we cannot agree on whether the fetus is
human or not.  If it is, then abortion is wrong because another person is being harmed. 
It it is not, then there is no crime.

One reason, then, is
because some actions have clear victims and others do not.