Gun Control In America

I have a question, but before I asked it I have to give you some background. 

On Sunday, 29-year-old Omar Mateen, a Muslim American of Afghan descent, went into a gay nightclub in Florida and began shooting. Before he was killed by police, Omar had killed 50 people and injured another 55. 

From reports I read, there are doubts whether Omar was inspired by Islamist extremism, was homophobic, or whether a combination of both caused him to do what he did. His ex-wife had said he was bipolar and had a history of steroid use, so he might just have been mentally ill. No matter the reason, one thing is without doubt. 

Omar Mateen would not have been able to kill and injured so many people if he did not have access to an AR-15 semi-automatic rifle. 

I have to ask why are such guns allowed in American society. An AR-15 is an assault rifle and as of 2012, there are an estimated 2.5-3.7 million rifles from the AR-15 family in civilian use in the United States. That is just ridiculous. Some armies in the world probably wouldn't have anywhere near 3.7 million rifles in their armouries.

Simple common sense tells you that if Omar did not have access to guns, he would not have been able to cause as much damage as he did. So back to my question, 

"Why is there still a debate about stricter gun control in America?"