Hm, interesting. I ordered billboards and signs printed on industrial ones, meter and a half in widht, like 7 years ago. They too had just CMYK iirc. Maybe things changed since that or they had them old. Or that photos in a close proximity to you can benefit from more complexity while for street signs it's just a waste of time and money.

I'm just speculating that increase of the tank count is directly tied to greed. Not only in making a picture slightly better, but selling you more inks and chemicals of more rare varieties. Basic CMYKs can be purchased anywhere, but distinct toners lock you deeper in their proprietary garden. That falls completely in line with what they did before.