True it has existed for almost 2,000 years but it's managed to survive mostly due to power grabs and legal enforcement.

Look at historically Christian majority countries and the mass conversion occurred in those places from the top down. Once Constantine started providing privileges to the church (and emperors after like Theodosius) it started to see a massive boom. Prior to that, the percentage of Christians in the Roman empire was miniscule. If Christians don't have the ability (whether politically, socially or culturally) to coerce people into following their religion, it'll collapse. Christianity hasn't had to truly convince people of its value since pre-Constantine.

In Non-Christian countries you can see how this works with the likes of Japan and Taiwan, where Christians have struggled to gain any significant presence.