I believe there were many reasons. There’s a very corporate aspect to human salvation, as with the fall. Through Adam man had rebelled, going his own way like a prodigal, being cut off from direct and intimate knowledge of God and all that relationship entails. But God began working with man immediately, later through a chosen people, revealing Himself and His will gradually down through the centuries as humanity was ready to receive it. The covenants played their parts, each one of them a step in man’s “education”, so to speak, in the big picture. The world would be ready, just barely, to receive the light when it finally appeared in its full brilliance in the person of Jesus Christ.
By then we’d experienced a great deal of time spent in a world where man’s will reigns, and the Master’s effectively gone away. We could develop a hunger and thirst for true righteousness, and be all the more ready to receive it, all the more ready to receive Him , the essence and source of all righteousness, when finally offered. Man’s own failure to achieve righteousness by his own efforts based on the Old Covenant was one critical lesson to be learned, a lesson we all need to learn as individuals. Man needs God first of all, above all else. “Apart from Me you can do nothing”, John:15:5. That’s what Adam didn’t get.
Other factors would play their parts. For example, based solely on reason alone, through philosophy, man had arrived at the understanding that a creator-God must exist In any case I think a certain collective human experience and maturation had to be in place before the time was ripe.
And, yes, the Church saw all history as divided by one critical event, the advent of Jesus Christ. Man would make a calendar based on that concept, interestingly, since it now dominates in world usage.