While the Empire had committed a number of serious atrocities since 19 BBY, Alderaan’s destruction was a major game changer. Why wasn’t that event referenced in Episodes V to VIII?
In short, because there was no particular in-universe reason to reference it.
I'm going to take the two pairs of movies separately. In The Empire Strikes Back and Return of the Jedi, the story follows the main characters, who hardly need reminding, and the hardcore supporters of the Rebellion - people who have already decided to commit their lives to the cause. There's no need to give them a rabble-rousing speech about how terrible the Empire is, because the rabble is already roused.
In The Force Awakens and The Last Jedi, on the other hand, take place quite a bit later, when the memory of Alderaan isn't as fresh and the link to the antagonists isn't as clear. Palpatine and Vader are both dead (to say nothing of Tarkin, who actually ordered the attack), and the First Order - though in many ways aspiring to be the Empire - is not the Empire. There's no reason to go dragging up thirty-year-old atrocities committed by a vaguely related government... certainly not when the First Order is perfectly willing to commit its own atrocities.
Perhaps more to the point, the focus of the movies was never on the politics or morale-building of a resistance movement. Instead, the focus was on the heroic escapades of the protagonists, who knew very well why they were fighting. Adding scenes about motivating the troops would simply have taken screentime away from the real purpose of the movies.
One reason for this is that the event ended so many lives that nobody mentioned it out of respect for those who died. In the Empire Strikes Back and Return of the Jedi, all of the rebels and the members of the Empire knew of its destruction and it was not mentioned as it was common knowledge. In the sequel trilogy, it was not mentioned due to the large time gap and other atrocities that occurred on a larger scale, such as the destruction of the Hosnian system.