There is no reason to produce the fuel.
Even without the on vehicle pollution equipment ULSD still lowers emissions - sulfur specifically - it is cleaner fuel. They want it in Mexico City to try to help lower the pollution. It lowers particulates etc. I think that is why it is written into the Hoy No Circula. It would not be done because there are vehicles in Mexico that require ULSD (in other words not for tourists from the US) but in an effort to reduce pollution. I would expect that once they have it they would require new vehicles built for Mexico to have the equipment US vehicles are required to have. I have no idea how to find out how far along the upgrades are or even if they are actually being done. I just posted that I read articles that said they are going forward with the modifications to the refineries. I have no idea what they are actually doing. Both diesel trucks and buses are built in Mexico for the Mexican market. Once they have ULSD they will build those vehicles there.