The question is: why do the Poison Traps location progress differently than the other two Primary locations?
The answer is multi-fold:
This gives players without Construction cards (or with low-valued construction cards) something useful to do. Remember, securing a secondary location gives everybody a global benefit but comes with a risk. Namely, the risk of being perceived as a Spy or Selfish Jerk. The Selfish Jerk needs Individual Reward Cards to win the game. The Spy can use the Individual Reward Card in a sinister manner.
This gives the non-loyal player with good Construction cards a means of discarding them. Go to the Traps, spend time progressing it, come back home, draw cards at the start of the next turn, discard your good one(s).
It gives players more to think about when an Invader shows up at the Poison Traps after the location has been completed.
I like the idea of the Spy escaping through the Poison Traps and being aided along by an unfriendly invader. Thematic, but it I like it.