Why Is that Frank? Because one loony toon psychopath, who didn't legally posess a firearm to begin with, broke the laws already in place to commit a murder. Do you honestly think that further restrictions to firearms would have prevented this guy from doing this crime, as he was willing to violate atleast 3 firearms laws before he even pulled the trigger. 
1. Possession of firearm with mental history. 
2. Loading a weapon on airport property.
3. Likely conceal/open carry laws that Florida has for having a loaded fire arm in public without a carry license. 

You may not know this, but laws do not prevent one from breaking the law. Additionally, CBS just reported that this guy tried to turn himself in to the FBI last November for being forced to fight for Isis. Either he's crazy or he's a terrorist, in either case he wasn't legally allowed to posess a gun.