'thul's story project...

[FoolishGirl]

Not quite decided on a true "why" to explain it. The reason these beings brought it in is to explain why there's changes in the bodies between generations, yet the in-world source for this acceleration is still a good question.
There could be some cost, not decided fully on that yet.
Traits that are seen as "unwanted" in a group can happen to fade away, rather than persist. One disadvantage could be the occurrence of strange evolutions. The various sub-species all started out as such evolutionary changes, developing fully in the span of 6 generations or so.
One "Cost" on the cultural front is that humans are less trustful of those that are different. Where it is skin color on the world of Tellus, it is actual physical traits in the world designed here. Not decided yet what will happen if two people from separate human species decide to mate, what their children will be like. The biological cost could be there.

It will be interesting to see if your world follows the earthly rules regarding evolution (only faster) or if some other rules entirely are in play! Perhaps there is some magical force (good or evil or indifferent) at work... I was thinking that if it's some kind of mutation-causing force (natural or magical), the people might show signs of illnesses like radiation sickness due to the constant DNA damage they undergo. Biology: fun!