One assumes because the religions themselves don't refer to Jesus or Moses that way, but Islam DOES refer to Mohammed as "the Prophet Mohammed" or just "the Prophet" all the time.
I should point out that the media regularly refers to Jesus as "Jesus Christ", even though "Christ" is a title, not a surname. I'd say that partially because Christianity refers to Him that way, and partially because they don't realize it's not a surname.
Jesus Son of God is very common. Find that in the NY Times. Maybe on their story of Jesus in a bowl of urine and calling it art.
Christianity also says Jesus is the Messiah, but the point is that neither is the title that's routinely appended to His name in conversation, the way "Christ" is, or the way "Queen" is routinely appended to "Elizabeth the II", or "President" is to "Barack Obama". By your logic, the NY Times should also be referring to the President as "Commander-in-Chief Barack Obama", but it doesn't, because that's not the title he's most often addressed by. "President" is.
If the NY Times did a story specifically about WHY Jesus is important to Christians, then they would make reference to the fact that He's believed to be the Messiah and the Son of God, just as when it does a story on Obama sending troops somewhere or discussing military operations with Congress or something, it makes reference to the fact that he's Commander in Chief of the military.
Titles are appended to names in the media according to the way they're appended in the source material, in this case, the Muslim culture.