The general public and to the government. The latter to make them think that no new (EU) legislation is needed to improve international ticket sales.
NS have the main ticket sales channel for international train journeys from the Netherlands. For European Sleeper, which they also refuse to sell, both NS staff and passengers want to be able to book through the NS international booking website. People keep asking about it. Belgian railways however don't see a problem and are selling European Sleeper tickets. Since NS use the same backend system, there's hardly any effort involved to start selling, but they don't want to.
I tried booking a European Sleeper via SNCB. They offer a choice between Seating, Couchette and Sleeper. However, the description they give for a sleeping car is "Comfort Standard with Washbasin", whereas ES say Comfort Standard is a Couchette car with reduced occupancy, and a sleeping car is "Comfort Plus". So what do you actually get if you book via SNCB a "Comfort Standard Sleeping Car"? Who knows.
My guess is that what they call "Comfort Standard" is actually "Comfort Plus" and booking the actual "Comfort Standard" via SNCB is impossible, which is a pity since it's the class I would prefer.
Even after many years of supposed close cooperation between DB and SNCF on FR-DE high speed services, the ticket sales process is unsatisfactory, as DB will quite happily lead you through the entire booking process for compulsory reservation trains with no available seats only to tell you at the last possiible minute that an "An error is occurred", whereas SNCF can show that at the first step in choosing a train.
Then what about GoVolta and their "book the seat opposite you" gimmick, or FlixTrain and their "book the seat next to you" equivalent, and... and ...
I support showing competitors in the journey planner, and providing necessary information to make a booking, and also that passenger rights should be available between the seperately booked tickets, but the idea that every train company should fully support every other operator's entire booking process within their own online ticket sales engine seems to me completely impractical.