One cause is that France is rather obstinate when it comes to sleeper trains passing through, but not stopping in, France. A direct line from the Netherlands and Belgium to Spain (e.g., Barcelona) would be viable and possible on standard gauge, but it wouldn't make sense to stop in Paris at 2:00 in the night or so. SNCF and the relevant French agencies insist on trains serving France as well.
Maybe some company will manage with a perfunctory stop in Perpignan?
The train passes through France non stop. Along the way, around 2AM, a single carriage decouples from the end and smoothly glides into a Paris terminal, where it functions as a de facto hotel room for the night as guests won't be kicked out until morning. Will anyone use it? IDK. But it does satisfy the requirement of stopping in France.
You van already do Bruxelles to Perpignan with a direct ( french ) TGV train, in 6/7 hours. If you want to go to Spain I believe bruxelles Barcelona via Paris is also quite practical. Everything is much pricier than plane and no night train though
I've done Paris to Barcelona on a sleeper to Perpignan and an express from there. I think for people who like sleeper trains, a through service would be far more convenient, even if it stopped in Perpignan early in the morning.
I suspect there is not a lot of track capacity across the Pyrenees, though, which would mean the sleeper train would have to get in before the morning expresses start.
Maybe some company will manage with a perfunctory stop in Perpignan?