They cannot advertise it (Mex Gobernacion rule) and ULSD costs considerably more than Mexican refined diesel. The station would become uncompetitive. Plus commercial diesels would avoid it like the plague if it became public knowledge. There is no easy answer.
Pipelines are (guess what?) owned by Pemex. They do not truck ULSD into Baja California. Comistibles are piped to Pemex tank farms then dispensed into pipas tanker trucks. If you think Pemex is going to pump a competitor's fuel for free......?!?
Now that the price difference of Magna and Premium has widened, transport services have taken liberties mixing cheaper fuel with more expensive grade. This has been true since the days of Pemex Cien and Super Mexolina.
In Baja California it is the huge distances involved that discourage the hauling of low sulfur diesel and Magna Sin to dilute into Premium. On the mainland, the barrier disappears.
I've no answer as to how to go about relieving all these barriers and traps. But for simple economics I have returned to purchasing Magna, then adding a bottle of Chevron Techron every 2,000 miles. Well, you can't have everything.
If and when ULSD enters a pipeline behind a pig, you can bet your butt it will be sent to D.F. first then to the state of Mexico. This is ironclad law and is going to remain a sore-point between the PRD party in power in the distrito and the PRI and Pemex. The regulation will be enforced meticulously for political reasons. The PRD will then publicly gloat "WE are the party that forced the cleanup of air pollution in Mexico City"