In a society with no fixed calender, "The Year of the Elephant," as it came to be known, was not only the most important date in recent memory, it was the commencement of a new Arab chronology. That is why the early biographers set Muhammad's birth in the year 570, os that it would coincide with another significant date.
But 570 is neigther the correct year of Muhammad's birth nor, for that matter, of the Abyssinian attack on Mecca; modern scholarship has determined that momentous event to have taken place around 552 C.E.
The fact is that no one knows now, just as no one knew then, when Muhammad was born, because birthdays were not necessarily significant dates in pre-Islamic Arab society. Muhammead himself may not have known in what year he was born.
In any case, nobody would have cared about Muhammad's birth date until long after he was recognized as a prophet, perhaps not even until long after he had died. Only then would his followers have wanted to establish a year for his birth in order to institute a firm Islamic chronology. And what more appropient year could they have chosen than the Year of the Elephant?
Reza Aslan in "No god but God. The Origins, Evolution, and Future of Islam" from 2005