Really there is only one answer which is agency (a.k.a free will) though this answer alone without context or understanding of how it fits into the cosmology is not a satisfactory answer.  
 
Heavenly Father is a just and good being perfected and incorruptible. He is the Father of our spirits and loves each of us, like any parent he only desires good things and wants us to become just like him.  
 
Since he is a loving and perfect being though he does this through long suffering, patience, and charity. He cannot force a single one of his billions of spirit children to follow his plan for eternal life and joy, for him to do so would be for him to cease to exist since he would become wicked. In fact forcing people to do what is right is the sin of pride that caused Lucifer to fall and rebel against Father's plan of agency and choice.  
 
One will ask why allow choice if choice provides so much suffering? Without choice there is no growth. Without choice there is no progression, you cannot force someone to comprehend anything, they have to learn through trial and error and because to this there will be mistakes and there will be some whom choose to do abominable things to others.  
 
Father knows this and believe me it pains NO ONE greater than it pains Heavenly Father to see his children suffer, but if he was to use his powers to force everything to be perfect now he is robbing us of eternal life and the ability to grow and become like him, which is the greatest evil of all.  
 
He DOES though provide the strength to face all trials and tribulations, if we will call out to him in faith. He also promises to restore unto those whom are unjustly harmed or treated 100 times the blessings and peace in the eternities to right the wrongs of the mortal existence.  
 
He cannot change mortality nor can he stop atrocity since these acts are done through the agency of mankind. We willingly do evil things to each other and Father cannot force us to stop least he damns us all to never be in his presence again.  
 
He does all he can he send prophet to teach us, his Only Begotten Son to die for us, and has a plan to allow us to return to him and partake in the blessings of exaltation should we be worthy through our faith in Christ and our works before, durning, and after this life.  
 
None of this makes any of the pains and sorrows any less profound, but think of this in your time of need. In the Garden where Christ prayed and started the process of the atonement the reason why he bleed from all pores is his literally took upon himself EVERY illness, injury, pain and sorrow simultaneously.. He felt it all and even time we hurt physically or emotionally he felt it all for all of us then with that burden upon him willingly faced his execution of the cross.  
 
So yes someone has been there has felt everything you are going though and understands your needs perfectly his name in the hebrew tongue is Yeshua, the Messiah, the Lord Jesus Christ in english.  
 
He want's to help each of us face these burdens and his uniquely is equipped to help carry us through since he has been there personally and triumphed over it all! 
 
I hope my answer can bring to light the reasons why Father cannot stop the agency of mankind, but how he does all he can in his love to help us face the realities of the world with faith and to grow to become more like him this day!

Suffering comes for many reasons. Some because of agency of another, some because of our own agency, and some for apparently no good reason at all. 
 
"It is often in moments of disappointment, heartache, and loneliness that make decisions that forge our faith, mold our character, and fortify our convictions. How do we know if we are honest unless our honesty is put to the test? How do we know we are filled with virtue unless there are opportunities to choose a non-virtuous path that we then resist? How do we know if we can bear under challenge and trial unless we have challenge and trial? And how can we expect to feel and taste the pure sweetness of the gospel of Jesus Christ, meaning specifically the power and peace of the atonement of Jesus Christ, unless there are times in our lives when we desperately need and seek that peace and power?" (No doubt about it, by Sheri Dew)

What dosent kill you, makes you stronger. Trials and suffering are there to teach us and guide us and god dosent give us anything that we cant overcome.