Christians in the Middle East largely converted to Islam over the centuries. However, there are still substantial Christian minorities in some Middle-Eastern countries.

Lebanon has a large Christian population, currently making up 40% of the population. Christians were, in fact, the majority religious group in Lebanon from its independence in 1943 to the Lebanese Civil War of 1975. Christianity is still the majority religion of the Lebanese diaspora.

In terms of absolute numbers, Egypt and Syria have the most Christians of any Middle Eastern country, although in both countries they make up only around 10% of the population.

Zealot by Reza Aslan (the guy who became popular for that horrendous Fox interview a while back) goes pretty in depth on this topic, albeit for a different reason.

One of the major themes of the second half of the book is the disagreements of Paul (who focused heavily on evangelizing to Jews in the Greek Diaspora and the pagans in Rome) and James the brother of Jesus (who very heavily focused his preaching on Jerusalem and the Jews of Palestine). Aslan focuses on how Paul and James had radically different interpretations on Jesus's teachings, particularly on who he was preaching for. Paul interpreted his teachings as tearing down the walls between Jews and Gentiles, hence the focus on the Greek Jews and, later, the people of Rome. James believed Jesus was a Jew preaching for his fellow Palistinian Jews and as a result took a very local view on the matter.

This starts to touch on your question with the sack of Jerusalem in 70 AD. As the Palistinian Jews were slaughtered or driven from their homes, James' congregants of the Middle Eastern church went with them. He viewed his church as a subset of Judaism, and when the Jewish population of the region dwindled, so did the Christian population.

Paul's version of the church, however, flourished thanks to his letters to far-away congregations and eagerness to preach to non-Jews, folks who weren't uprooted from their homes and forbidden to practice their religion. Aslan especially focuses on the fact that most scholars believe the gospels were written several years after the sack of Jerusalem, and posits that they were colored in a way that would make Christianity more palatable to the Romans of Europe. He calls out how the gospels make Pontius Pilate look like a magnanimous and just man whose hands are tied because of the common rabble, while most Roman accounts of his rule in Palestine color him as a bloodthirsty tyrant who wouldn't think twice before executing a Jewish peasant. Aslan says the authors of the gospels realized their most fertile flocks were in Rome, and wanted to focus on bringing them into the fold.

All in all, it really is a fascinating book and very much worth all the attention it got. If it's a part of history you're interested in, I can't recommend it enough.

Well, there are, Armenia, Georgia, Cyprus, and Lebanon (sort of, about a 50-50 split). Even then, Armenia used to be about 80% muslim until the russians colonized the area with Armenians, Lebanon had a similar thing happen with the french colonizing the area with chrisitans, and georgia and cyprus spent most of islamic history under christian control, but still retained significant muslim minorities none-the-less. However, the reason why Christianity is no longer widespread is basically Islam and it's incentives towards conversion. You had a lot of extra taxes and while you were allowed to worship freely, it was often a pragmatic choice to simply convert over to islam. When you are dealing with over 1000 years of Islamic history and dominance, you will simply see one person take that choice out of one of the 50 or so generations of people and it simply stick because you could be executed for converting away. One thing I disagree with the rest of the thread on is how long it took however. To get about 50 percent of the population to convert, it took about 400 years, with a standard deviation of about 200 years if fully control by a moderate islamic government without colonization efforts. I am basing this off of what happened with the balkans, as this is about 400 years of turkish control of the balkans, along with what happened with other regions, but that is only a back of the envelope type calculation.