Muslims keep asking why atheists only criticize Islam and not other religions. Christians keep asking why atheists only criticize Christianity and never other religions.
One might wonder if Christians and Muslims are inherently selfish and only think something is bad when it is done to them, and don't even notice when the same thing is done to others, but I don't think that is what is happening.
In a Muslim-Majority Country, Most of the Atheists Are Ex-Muslims
Hopefully, I don't have to explain why this is. If you are in a Muslim-majority country, then most of the atheists you talk to will be ex-Muslim atheists. There's two important facts in that previous sentence: you're in a Muslim-majority country, and the atheists you talk to are probably ex-Muslims.
In a Muslim-Majority Country, Most Bad Stuff is Done By Muslims
In a Muslim-majority country, most of the bad stuff that is done in the name of religion will be done by Muslims. First, this is because most of the religious people in a Muslim-Majority country are Muslim, but also because any time you have a religious majority, they are probably going to be doing various awful things to the religious minorities in that country, things that range from annoying to human rights violation that the rest of the world needs to be told about, and you can bet the former will greatly outnumber the latter no matter the country.
Even in a country with supposed religious freedom like America, you have all kinds of petty things happening with local governments, such as city hall trying to make it harder for non-Christian religious groups to build or expand places of worship, not to mention varying levels of verbal abuse, employment discrimination, etc. directed at some or all religious minorities.
So not only is most of the bad stuff done by religion in a Muslim-majority country going to be more likely to be done by Muslims, but the majority treatment of minorities is always going to be a bigger concern than minority treatment of the majority because the minorities lack the social and political power to do any real harm.
I get that if you are Muslim, you believe that Muslims are only good and only non-Muslims are bad, but the reality is, no matter what country we're talking about, most of the bad stuff done in the name of religion is going to be done in the name of whatever is the majority religion. Your desire to believe otherwise is irrelevant to your ability to prove otherwise.
Ex-Muslim Atheists Are More Familiar With Islam
An ex-Muslim atheist is going to be very familiar with the Quran, the Hadith, and the various arguments Muslims make in defense of Islam and the content of their scripture. An ex-Muslim atheist is not going to be as familiar with the holy books and apologetics arguments of other religions, and it is not reasonable to expect them to become familiar with those things, particularly given that most of the pro-religion arguments being made in a Muslim-majority country are being made by Muslims.
There is no reason for an ex-Muslim atheist to learn about the bad excuses Christians use to explain away the pro-slavery passages in the Bible and there is no reason you should expect an ex-Muslim atheist to take time to study such things. To request such a thing is unreasonable, and an obvious attempt to stop talking about what is wrong with the arguments made by Muslims, which again, are the most common arguments in any Muslim-majority country.
Conclusion
It should go without saying that everything I said about Muslims in Muslim-majority countries is also true about Christians in Christian-majority countries. I myself am American, and thus even though I was not raised Christian, I'm much more familiar with Christian beliefs, the Christian scriptures, and Christian apologetics than I am with any of those things from Muslims, Hindus, Buddhists, etc.
While I do find apologetics debates interesting from a purely academic point of view and will engage in apologetic debates with Christians, I must confess that my eyes glaze over when people want to talk about Christian "fulfilled" "prophecies." I simply do not have the interest to study any of that topic and am happy to leave those arguments to ex-Christian atheists, who often know a lot about that particular subject.
When I argue with Muslims, I'm going to stick to the most general discussions (e.g. have Muslims proved that their god exists), and I'm simply not going to get into debates about specific details from the Quran or the Hadith because I'm not qualified to do so. I'll leave that to the ex-Muslim atheists.
Since this is a blog in English, I assume that any theist reading this is more likely to be Christian. If you ever wondered "Why do atheists only criticize Christianity and never Islam," then you will need to learn to speak Arabic or Farsi and visit the Arabic or Farsi parts of the Internet to find out that you're wrong about that. If you wonder why atheists in Christian-majority country almost exclusively talk about Christianity, you really should not have needed me to explain this to you.