I remember reading that many financial institutions weren't keen on offering the NISA because of the low cost structure. For a few years at least, they expected to lose money by offering them.
In the UK where the fees are actually higher than Japan, maybe 5 years ago the platform I used suddenly introduced additional fees because of the wafer thin margins.
I guess in the coming years there will be a shake-up with NISAs if the financial guys can't make money. This could mean higher fees or fewer providers offering them.