It's easy to buy a 3.5in USB floppy drive for $10 online. In contrast, I've seen 5in USB floppy drives, but they're not common. For example, this is a USB controller for $50 - you have to hook up your own drive and provide a housing.
It's unsurprising there's less demand for them than 3.5in drives, but it seems like the difference in demand alone can't explain this gap. Is there some technical issue that makes manufacturing drives like this difficult? Or am I missing some other part of the picture?