Well, maybe because this was an American citizen that committed these acts and not anyone associated with these groups you've mentioned.  

Or the fact that less than 100,000 refugees have fled their homes, struggled in camps and have went through the grueling process to be granted asylum in the US since 2012. 

In fact 11 out of the 12 worst mass shootings in US history were perpetrated by us citizens. A Vietnamese immigrant was the only exception. 

So to discuss any of the misinformed topics you've stated would be out of place, inane, and wrong. None of them have anything to do with the situation at hand.