Many prophets are mentiond in the old testament, and they are all flesh-and-blood (btw, Muhammad is also flesh and blood). So why did God deliver his message through Jesus (God's "manifestation"), instead of sending another flesh-and-blood prophet like he used to?
Why God delivered His message through Jesus, God's manifestation, instead of using a prophet like He had at other times, is because what Jesus delivered was more than just a message.
Jesus actually became the message, and He was the only one qualified to do so, because the requirements for this job had to be sinlessness, and not just sinlessness, but the perfect sacrifice to satisfy the debt owed on behalf of all of humanity and their sin. He not only had to be sinless, pay the penalty, but He had to redo what Adam undid, He had to be a man and reverse the curse by never choosing sin, which Jesus also did.
Since sinlessness is unachievable by humans, God sent Jesus.
Jesus paid for this debt with his own life and blood, and the Father accepted it as just payment for the sins of humanity for all time.
"for He (Jesus) who knew no sin, became sin for us, so that we could become the righteousness of God in Christ Jesus." 2 Cor 5:21 Bible
Before Jesus, "flesh-and-blood" monotheistic prophets could only address God through what later was termed Apophatic theology. The "introduction" of Jesus brings in what was later termed Cataphatic theology. This significant change made the concept of God much more accessible to humanity than before.