"Nordic cuisine" is probably used because it provides better name recognition too, in addition to a wider selection.

Karelian stew and mämmi. Crispbread to go with them. That’s it.

It's much more than that. https://fi.wikipedia.org/wiki/Suomalainen_keitti%C3%B6 But a lot of those aren't internationally known or specifically associated with Finland so it doesn't make it worth it to run a specifically Finnish cuisine focused restaurant.