I always thought that most of those chasing the traveller (the so called "darkness") were previously uplifted races/fallout from previously uplifted races. The Fallen are somewhat greedy and reckless and much of what they make seems to be kind of ad hoc (destiny 2 has a mission where they are making an explosive to kill the hive and your ghost comments about how sloppy it is) once uplifted they continued this way and divided into competitive houses so the traveller abandoned them. Being the way they are they won't stop until they get it back or get as many pieces as possible back. The Cabal once uplifted destroyed and conquered throughout the galaxy and this is why the Traveller abandoned them. They're mentality of conquest has them follow the traveller to use it again, and to conquer any new systems it finds. The Vex are machines made by an uplifted race not unlike the War Minds or the Exo except something went wrong and they wiped out their creators or once the Traveller left their creators race they were sent to pursue it for them. The Hive is made mention of coming from a different dimension and crossed into ours when barriers between the 2 weakened or overlapped can't 100% remember but that may have happened as a result of an uplifted races reckless experimentation with new tech destroying them when the hive poured into their world and the Hive now wanting to corrupt the power of the traveller for their own use