This is a theory behind why secular historians did not mention the Bethlehem Massacre — Herod the Great was such a monster that to record every massacre or other act of murder on his part would have required several scrolls. (He even killed members of his own family, for Heaven's sake! Repeatedly!) This is probably the best counter-argument to the charge by Herod's defenders that St. Matthew made the story up to discredit Herod.
There's also the theory that it was recorded at the time, but as time went by and records were lost, it wasn't considered important enough to remember, again because of all the horrendous crimes Herod committed.
To give some further perspective on that: the year Herod the Great may very well have perpetrated the massacre, ca. 4 B.C., was also the year he immolated 2 religious teachers and about 40 youths over a religious conflagration that flared up in Jerusalem, and executed his son Antipater II. It was also the year he died, after which his son Herod Archelaus took control of Bethlehem and the surrounding area. Archelaus, for his part, began his reign with the massacre of an estimated 3,000 Israelites for opposing his ascension. In view of these atrocities, the overnight slaughter of a few babies in Bethlehem (the most reliable estimates range from about 4 to 20 babies) could easily have gotten lost in the shuffle. Naturally, scholars tend to assume it was all these massacres which were eventually distorted into the Bethlehem Massacre.
Furthermore, Bethlehem was a small rural town, and might not even have had all that many families with children that young in the first place.
For Christians, the Trial of Christ was a defining moment of the faith. For Pontius Pilate, it was just another day of clearing his docket by trying another self-proclaimed prophet/Messiah. While Pilate probably did remember the trial, it being one of several incidents that nearly landed him in hot water with the increasingly paranoid Emperor Tiberius, and a political hot potato he'd made every effort to dump into Herod Antipas' lap instead, it probably didn't rate any higher in significance in his official Acta report to Rome on the year's events than those two highway robbers he'd also crucified at the time. All of Pilate's Acta are now lost to us, having evidently perished along with a great many other bureaucratic records with the fall of Rome.
A couple decades later, Suetonius mentions that Emperor Claudius dealt with some "Chrestus Riots" in the city of Rome by expelling a number of Jews. These very likely refer to arguments between Jews over this new "Chrestus" (Christian?) sect coming to blows, but we can't be absolutely sure. Suetonius didn't see fit to say anything more about these riots, since he and most Romans knew that Jews in Rome were always squabbling over something.