"So many"

This is going to be difficult to answer because so many Lebanese settled in so many places.

So, for example, while there are 400,000 Lebanese-Mexicans, there are 7 million Lebanese-Brazilians, 1 million Lebanese-Americans, 100,000 Lebanese-Cote d'Ivoireans, 300,000 Lebanese-Australians, 100,000 Lebanese people in the UAE, and so and so forth ad infinitum.

In general, like the Irish and the Jews, the Lebanese are a diaspora community par excellence. I'm not sure there's much use in asking why "so many" ended up in any given place beyond that place being what was willing to accept them during the periods of mass Lebanese migration.

Edit: except, that is to say, that I think the more interesting question, as with the Irish and the Jews, is why so many of them were forced to leave their place of origin in the 19th and 20th century. But in that sense it's Lebanon/Syria that should be the focus of the question, not Mexico.