May I be so bold as to suggest that it just might be because Jesus doesn't get a mention in the OT?
You are distantly thinking along the same lines as I am. There are two religions (not counting small cults founded after 1800) which feature Jesus, one of which currently or previously has depicted Jesus in different ways.
The New Testament as a single compilation did not exist until the 4th century after the Council of Nicea in 325. It is believed that Athanasius compiled the New Testament in 367 and it was commercially available around 390. Prior to the Council of Nicea, Christians used many books and documents in addition to the Old Testament and only a small fraction of them made it into the New Testament.