Exactly, and it's not just about reading but also (perhaps even more so) about writing. If Nintendo control the manufacture of their proprietary disk, well, it's unlikely that a pirate outfit will be able to build a disk manufacturing factory that produces something compatible with retail Wii U's they can then sell their own non-Nintendo licensed games on. Or burn Nintendo's own games onto. The cartridge era of games were much easier to reproduce the physical medium onto, and this was super-widespread.
This did break down for Sony later on in a way it hasn't for the Nintendo Switch, a widely shared opinion on why the PS Vita flopped was because it was incompatible with standard SD cards _and_ that their own flash game cart and memory card offerings were so incredibly expensive. (And there were cheap SD-to-PSVita adapters that were released anyway. [0])
This did break down for Sony later on in a way it hasn't for the Nintendo Switch, a widely shared opinion on why the PS Vita flopped was because it was incompatible with standard SD cards _and_ that their own flash game cart and memory card offerings were so incredibly expensive. (And there were cheap SD-to-PSVita adapters that were released anyway. [0])
[0] - https://en.wikipedia.org/wiki/PlayStation_Vita#Hardware