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?
Maybe some company will manage with a perfunctory stop in Perpignan?