Several reasons....
1) Murder rates tend to be higher in the 'bad' parts of town. Conversely, voter registration is lower in these areas. In other words, people that vote tend to live where murder is not a 'hot' topic.
2) The day to day things that prevent murder (i.e. more cops on the streets, better enforcement and coviction rates, etc...) are controlled by state budgets, not federal.
3) Murder rates haven't really changed in the past 10 years.
http://www.deathpenaltyinfo.org/article.php?did=169&scid=12
In any society, you have to balance things. Unemployment, homelessness and child abuse are some other things that we will never eradicate. We can only fight them to a 'draw'. That's not to say that we should give up on trying to change them, but there is a point of 'diminishing returns', where you just spin your wheels, wasting time and money on a problem that is impossible to entirely solve.
As long as there are people, there will be murder. And theft and rape and drugs and hunger and pain and suffering.
These are issues that the canidates won't talk about because there are no votes in trying to solve these problems.