Burning books like this requires enough targeted hatred and a need to offend the targets. It also is usually more effective if your target is a minority (not specifically racial, just a much weaker target), as that bolsters your position of power and lessens the likelihood of retaliation.

Atheists sometimes have that much hatred, but at least where I am Christianity isn't small enough to be a minority, so the fear of backlash might be holding back some of the hatred type atheists.

Another reason may depend on why someone became an atheists. Many of us don't hate religious people directly, we just have issues with what organized religion gets away with. People like me came to this conclusion by comparing scientific evidence to blind faith. By nature, faith will be more emotional and reactionary, whereas if you come to a position after learning and changing yourself, you're more open to understanding why it's not really a good idea to hate like that.

Though I'd call myself agnostic vs an atheist.