Possible reasons vary, many of them varying in degrees of plausibility, and almost certainly there was a conjunction of reasons only some fraction of which we will be able to discern. One reason may be that the human population index exploded around that time, so that although human beings have been around for more than a hundred thousand years (give or take), about 98% of the human beings who have ever lived have lived in the last two thousand years (see
ourworldindata.org/world-population-growth/). Christ came just prior to the human population explosion and was situated in a time and place where his message would feasibly reach the greatest number of people. It wasn’t so pre-historic or primitive as to be inevitably obscured through history, but also not so late as to be effectively provincial in its sphere of influence. It was late enough that we had developed the discipline of history, had created the written record (etc.), but not so late to the party that we already had camera phones. The fullness of time indeed.
Christ appears at a moment of history which can be empirically investigated, and we can be reasonably certain that the basic outline of Christ’s life is historical - but he is not so evident in all respects to the historian that the historian cannot avoid being Christian. All the reasons God has for being relatively hidden may also motivate his incarnating himself on that historical periphery, where he is accessible for anyone willing to seek, but not so forcefully evident that nobody can run away.
God may also have known how people in any age would have reacted to him, and so knew that at no time (earlier or later) would as many people have accepted him in the long run if he came to us then as if he came precisely when (and where, and how) he did.