Because it won't. Here's the recap of Colbert's monologue from vox.com (reposted from here yesterday).
Stephen Colbert is exasperated with America's unwillingness to do anything about its unique gun problem.
Breaking the comedic tone on his show on Friday, Colbert echoed what's now a common theme in the discussion about guns: Why doesn't anything change?
... pretending is part of the problem. These things happen over and over again, and we are naturally horrified and shocked when we hear about them. But then we change nothing, and we pretend that it won't happen again. Some say the answer is stricter gun laws. Others say the answer is mental health care — that we need better treatment or just keep the guns out of the hands of the insane. Maybe it's both. I honestly don't know. But I do know that one of the definitions of insanity is changing nothing and then pretending that something will change.
So the first thing is to know that nothing will change unless we - American citizens - take motivated action to change it.