Because in real life, MP5s get a LOT less use that you probably think. the organizations that use them tend to be smaller, especially when compared to the proliferation of m4s and AKs. Also, The AK has been constructed in many, many countries, and different time periods; and magazine type has changed with both of those.
To the best of my understanding, MP5s are on the way out in a lot of organizations in favour of M4s which are only marginally bigger with a lot more supply of parts. Specialist units that want rounds that don't penetrate as far as a 5.56 are the ones that I notice seem to want to keep the Mp5.
In short, there's a shit tonne more M4s or AKs out there, and therefore a bigger market for different kinds of mags.
There's other reasons, but that's the gist of it.