Because Christians think they have the right to impose their beliefs on everyone else by any means necessary--for example:
Anti-gay laws, homophobia, anti-gay hate crimes such as beatings/killings of gay people and bombings of gay night clubs.
Anti-choice laws, sl*t shaming of women who merely want to control their bodies, terrorizing women who seek reproductive health care, bombing of women's clinics.
Misogyny
Stifling of scientific progress
Child rape by clergy that is covered up, denied, and ignored by churches.
Demonization of atheists, discrimination against atheists, death threats against atheists, etc.
Mutilation of children's genitals because "god" commands it.
Prayer being forced on us with taxpayer dollars by government officials, taxpayer funded monuments to religion on public land, "faith based initiative" which funnels taxpayer money to religious groups.
Etc, etc, etc.
And they cry "persecution" when they're told otherwise--particularly when atheists step in and ask that the Constitution (which demands separation of church and state) be upheld and the rights of all be respected.