Why do Chimera Ants seem to remember only one past life?
From what I read/seen in the anime/manga is that they remember the memories associated with strong emotions. I.E Colt has a strong sense of protection because in his human life he was dead set on protecting his little sister. These emotions probably cause the memories to take precedence over others. There are how ever small instances that show both sides can be remembered. There is a conversation between Killua and Ikalgo, in where Ikalgo recalls his time as an octopus and always envied the graceful squid. Later when Ikalgo is with Welfin in the control room after shooting Welfin in the legs Ikalgo makes him speak of his human memories. This conversation jogs Ikalgo's memory and he starts to remember some of his past human memories as well.