There is a good podcast by planetmoney explaining it. Mostly it is because there are so many industries and interests touched that regardless of what you negotiate, there will be public outrage and pressure on the negotiators which make it much harder to reach an agreement. If that is the case though, I am not sure whether an agreement must be reached...
Anyhow, interesting podcast: http://www.npr.org/sections/money/2015/06/26/417851577/episo...