Right now there is a huge transport bottleneck between Iran's Caspian Sea ports in the north and Chabahar in the south of the country. There is a lack of rail transport links, and the terrain is quite hard to build through, basically you have mountainous terrain and any railroad in the north would need to have high grades and require high power locomotives. Most cargo will actually go around the Caspian Sea via Kazakhstan and Turkmenistan, possibly via Azerbaijan later. There are plans to make rail links to Iranian Caspian Sea ports but that will take a long time to materialize.
Given the current Iran approach toward China, I don't see much help from BRI on their infrastructure. On the other hand, China now has built up presence through Caspian Sea and Kazakhstan + Turkmenistan and Azerbaijian. Once that infrastructure is built up, you have a reliable middle corridor. there is just no incentive to build another one through Iran.
Countries that want to work with China needs to understand that china has finite resources and demand. It cannot take losses on every infrastructure project that it invests in. If you move slow like Iran, then you get left out.
Also, you can't include Chabahar as part of CPEC, since it would hurt Gwadar.