Because their government functions on anti-western rhetoric based on the obvious bad blood.
The nuclear deal was isolated from practically everything because Iran has it's own regional goals that run counter to US interests (Saudi interests), and having deals based on taking away regional sovereignty is an automatic no go.
It's why the Iran Nuclear Deal was perfect, because it was just the Iran Nuclear Deal, and didn't involve things like state sponsoring of terrorism or ballistic missiles.
Iran's "negotiation" is basically to get the US out of the entire region, and that's an auto no-go.