After the Crale weapon of mass desctruction called the Sethignis accelerated the natural process on the Mer home planet which created new volcanoes with terraforming, waters became polluted, and there were very frequent earthquakes, natural life on the planet nearly entirely died out, thus the planet slowly became unhabitable.
Many of the Mer successfully evacuated their planet but - as they didn't have enough time and their knowledge of space travel was very new, billions have died.
After the Sun Emperor's secret was revealed (possibly by his own son, Tahebyu, who previously tried to stop his father's cruelty), most of the Crale rebeled, and it ended with the Sun Emperor's death. Tahebyu, after becoming his successor, realised the gravity of his father’s deeds and made a treaty with the Mer and he offered help in creating the remaining of the Pari System.
After their life were not in apparent danger the Mer started looking for another planet they could live on. They called this the Eden Initiative. Their desperate search eventually led to them becoming somewhat of an “explorer race” and thus they also have a huge collection of archeologic finds.