The one book, Alien: River of Pain, that describes the Xenomorph infestation on the Acheron colony doesn't actually explain why but it does contain this exchange after the infestation has already claimed several lives including that of Newt's dad:
Most of the morning had come and gone and there hadn't been an attack from the aliens since the middle of the night.
"You sure they're not going to attack during the day?" Stam asked.
Dr. Reese frowned. "Not certain, of course. There isn't enough data. But aside from the 'births' of the newborn aliens, their appearances have mostly come at night."
"Mostly," Stam echoed.
(Alien: River of Pain, pp.252-253)
In this case, most of the colonists weren't being killed by the Xenos at the time but were abducted to be impregnated by facehuggers. The main attack that wipes out, or abducts, most of the colony including Newt's remaining family occurs within five hours of the exchange between Stam and Dr. Reese, which according to the timestamps in the book is between 2 and 6 PM on Acheron.
As for why the Xenos attack mostly at night, since most of the colonists were hunkered down inside the Hadley's Hope facility it may have been easier for the Xenos to catch them off-guard since, at the time, they were mostly concerned with A. obtaining new hosts and B. protecting their hive from an existing Colonial Marine force already present on Acheron. A character in the novel does mention that some of the colonists blame the lack of coordinated protection efforts for the loss of the others, and it's not until fairly late in the infestation (which starts and ends in roughly 72 hours) that they actively relocate to a single unfortunately-not-so-secure area in the colony where the Xenos have to make a concerted effort to attack.
However, the novel doesn't describe any of the 1-month gap between the fall of Hadley's Hope to the Xenos and the arrival of Ripley and the Marines from Newt's point of view, so we have no way of knowing how often the Xenos came into the colony during that period.