Simple. Because when some white right-wing lunatic blows up a building or shoots up a gathering or does something else of the sort, nobody goes around physically attacking whites. Nobody seriously suggests deporting them, or outlawing them, or reeducating them, or passing laws that treat them differently from others. Nobody suggests the First Amendment or the Fourth Amendment (or the Second, for that matter) doesn't apply to them.
And on those rare occasions when someone does stupidly overreach toward the Right and/or Christians with a broad brush (cf. that US Army Reserve PPT I mentioned yesterday), the Right is there in the forefront slapping down any such suggestion (but always only about the groups they represent or like or aren't scared of — if it's about Muslims, well, then, it's still more proof that We Have To Do Something About Their Typ ).
I'll speak up whenever I see someone being grouped unfairly with violent extremists. But if it seems like the responses to different classes of people being violently extreme are asymmetric, that's because there's plenty of asymmetry going on in the other direction.
Why the Media Treats Right-Wing and Islamist Terrorism Differently
Racial and ethnic minorities are often made scapegoats. The press ought to be attuned to that.
I had a cousin who was insisting that there was no difference between George Tiller and Kermit Gosnell. I was shocked that I had to point out why the media handled the two cases differently: one was murdered by a radical Christian. She didn't even think that was worth batting an eye at.
There are a lot of other differences as well, but that's a key one.
Don't worry; I gave her a list. She didn't heed them, mind you, but I gave her a list ;).