Not sure if people are aware but the Rakghoul event was the event the led into the Kaon Under Seige and Lost Island FP. It was a one time deal. Now that Dr. Lorric is dealt with, Bioware would have to create another story arc to describe why the Rakghouls are back.
As to the Chevin event, that was event that led into the Gree's return to known space. Now that the Gree have returned, why would we need another event telling us of their return.
The Gree even is reoccuring due to the fact that the Gree arrive at Illum have us do a little research for them and then leave. They return to get more research done.
This is why the Rakghoul and Chevin event are no longer available, and the Gree event is reoccuring.
In 2.3 we will have a new event (Bounty Hunter Contract Week) that should also be reoccuring.
You can say you want the Rakghouls back, but as I posted there is no story arc to warrant it.