There's two ways to answer your question, depending on what you mean.
Lebanon, as a modern independent country, is more Christian than its neighbors because it was specifically designed to be so. While from the end of the 19th century, and in large part due to foreign pressure, there was an adminstrative division granting a quasi-autonomous "Mutasarrifate" of Mount Lebanon which would have been overwhelming Maronite, this was purely an administrative division. You can see what that looked like here: https://en.wikipedia.org/wiki/Mount_Lebanon_Mutasarrifate#/media/File:Mount_Lebanon,_Ottoman_Syria_1914.png
Note that this unit excludes Beirut, not to mention Tripoli or Acre, and therefore lacks a viable port. Moreover, it was an administrative convention. It was no less an interconnected part of the wider region than the Mutasarrifate of Jerusalem which was created for similar reasons. Culturally, it was a predominantly Catholic Christian subdivision of a "Greater" region that had other, comparable pockets of religious minorities including the predominantly Alawite Latakia, or Jebel Druze.
By the end of WWI, we know from the King Crane Commission that while the majority of people in Greater Syria (about 80%) wanted a united independent Greater Syria. There was majority support among the Maronites of Mount Lebanon for some kind of independent Christian state under a French mandate, but in the absence of foreign intervention this was unlikely to have ever come about.
The French, however, who were the responsible mandatory power for what would eventually become Syria and Lebanon (along a line divided from British control to the south by the Sykes-Picot Agreement), were more than happy to oblige.
The debate, then, became what such a state would look like. The French foreign ministry was initially skeptical of the Maronite nationalist claims for a "greater" Lebanon that would have established their control over a large number of territories beyond Mount Lebanon that were wholly non-Christian. Ultimately however, the decision was made General Henri Gourard, a devout Catholic and a believer in the Maronite cause who was appointed administrator of the French Mandates.
Gourard carved out a Greater Lebanon that was essentially as large as possible, including the fertile farm land of the Bekaa Valley and the south and the ports of Beirut and Tripoli, while still remaining majority Maronite. The system the French established, with cooperation from the Maronites, came to be known as "confessionalism" in which the proportional governance of the state would be determined a proportional basis by sect. This ultimately formed what in Lebanon is still known as the "National Pact", which among other things stipulates that the President of Lebanon must always be Maronite, the Prime Minister Sunni and the Speaker of Parliament Shia, and so on down the line for Lebanon's seemingly endless array of smaller sects and schisms. As the Maronite population, proportionally, declined in number in the 20th century the demands of minority populations for greater political power would ultimately provoke the Lebanese civil war, the conclusion to which (the Taif accords) substantially altered the balance of power in the country, for instance by weakening the powers of the Presidency and changing the proportions in Parliament.
So that's how Lebanon came into being as a majority Christian country, without which it might easily have simply been a majority Christian region of a larger Syria.
If your question, on the other hand, and to get to the second part of my answer, is why that portion of Greater Syria was majority Christian at all, well that's a bit harder to answer, but basically it's a bit chance, a bit geography, and a bit history.
Going back to the Islamic conquests, Mt. Lebanon had been an incredibly difficult portion of the region to pacify. In that sense it closely resembled Armenia which benefited from similar conditions of a mountainous area defended by a war-like people who were more than happy to wait out any trouble. To this day the common refrain of the Lebanese when you ask them what they would do if there was another outbreak of fighting is just "Oh, I'll just go up to the mountains."
This part of the Middle East seems to be particularly conducive to the long-term survival of an incredibly diverse group of religious sects, more or less throughout its history, including Christians (Maronites, Greek Orthodox, Melkite Catholics), Jews, Sunnis, Twelver Shia, Druze, Alawaites, and so on.
The Maronite community, in particular, also benefited from relatively early trade and political links with other Catholic powers, particularly France. France would end up basing part of its claim to Greater Syria in the negotiations during Sykes-Picot to their occupation of the region during the crusades. As I already mentioned, the creation of the semi-autonomous Mount Lebanon region in the 1860s was partly the result of French pressure.
But that's not to say of course that this was only patch of the Middle East where such a thing was possible. The Copts of Egypt, who had none of these advantages, are to this day about 10% of the population. Before Turkish independence (and the pogroms and ethnic cleansings that preceded and followed WWI) the Turkish coast around Izmir (then Smyrna), for example, was a majority Greek Christian enclave, so much so that in the original treaty partitioning the Ottoman Empire, the Treaty of Sevres, it was to be granted to the Greeks outright.
Source wise:
I would have a look at D.K. Fieldhouse's Western Imperialism in the Middle East first and foremost.
The King-Crane commission reports are available online, but, for instance, this article is a decent summary of their findings.
For regional context across a longer period of time, Eugene Rogan's The Arabs: A History is excellent.
