Most simplest answer is because we are Muslims first and foremost. Not that anyone else is being barred by it. It just is. We are compelled, as Muslims, to bring the news of Islam to all humanity anyway, it's just that, pain for me to say, many of us Muslims are being astray, myself included, from the path of Islam. So, we call upon our brothers and sisters first. Hopefully, after that, we can continue the responsibility to preach Islam as the extension of our beloved Messenger. But, feel free to call upon Christians, Jews, atheists and whoever else. It's a free protocol anyway. No one is stopping y'all.

We are human first and foremost. That what seems to divide us was put upon us as a psyop for ages. Allah, God, Buddha, the Spiritual Energy, the Higgs Field, ... is here to help everyone on his path to paradise. Yes, I am a believer.