TW: I am going discuss some of the specifics of Sandy Hook.
Just conjecture on my part but it probably was because Sandy Hook was a little unique in that there were twenty victims between the ages of 6 and 7 and the shooter was 20 years old. Many American school shootings involve a teenage or young adult perpetrator targeting victims that are of similar or slightly younger age. I suspect that makes it easier for some sociopaths to dismiss these incidents as potentially retribution for bullying or interpersonal drama.
This incident was unique in that it was just a grown man murdering little children. There is absolutely no way to blame the victims in any way, shape, or form. There is nothing a 6 year-old could possibly have done to cause this. Not only that but it holds the ignominious distinction as the deadlist shooting at an elementary school.
So I suspect they targeted this incident in particular because they feared it was going to serve as a powerful catalyst that would sway public opinion in the debate on gun control. By trying to convince people that it was not real they hoped to decrease the impact it might have had.