There is nontrivial cost in recording the alternate audio tracks. And ideally, there may be some subtle editing of the visuals as well to keep the lip motions from getting too far out of synch. And while adding anyone language isn’t that big, adding “a lot” starts adding up.
On the other hand, text takes even less space, so In Theory adding more subtitle tracks ought to be much easier.
However, there’s the question of whether the DVD standard anticipated that much flexibility. There may be hardcoded limits. Or there may be issues of how much information can be interleaved/buffered how well, to avoid seek delays across the disk.
Remember, as with any piece of consumer hardware, these boxes and disks were designed to be Just Barely powerful enough to Just Barely handle the worst case someone anticipated at the time. When you start asking for more, you’re looking at a more general computer and cost goes up. Admittedly at this point the hardware to meet those basic requirements is a lot cheaper than it used to be – but the standards may not have been expanded to permit taking advantage of that.
That’s actually the best argument for “generational” changes like DVD to Blu-Ray; they’re an opportunity to redesign those standards for greater flexibility. Or they should be. Whether anyone actually does so – given that the drive to optimize the machines still remains – is always an open question.