It talks about the trading prosperity/failure with Happy Trails. Your contract ended after a certain incident. Therefor the outcome of the trade has no effect on you.
On the matter of why go back. Zion is a haven for hardcoe Players, meaning there food resources there are so abundant you can basically live in Zion on hardcoe mode and you'd almost never have to leave. (Only when you've lotted every resource and need to wait for them to cycle/spawn)
They didn't do it with Dead Money because the whole theme behind the DLC was Letting Go. Kinda hard to bring that across as a viable theme when you keep coming back. :shrug:
Please Note: I'm not trying to denounce your opinion, I'm just stating mine on the matter.