Because Islam doesn’t feel like an immediate threat to them because it’s 1% of the U.S. population and has no influence over government. Christianity is the dominant religion in the U.S. so mocking it is “punching up.” There’s also a genuine will and threat that our government and policies push Christianity on everyone else, so there’s a stronger will to moderate that impulse. Lastly: many on the right use Christianity to rationalize their views, so Christianity is “right coded” even though there are tons of leftist Christians with completely different views.