Experienced the same thing on my first city. It kept me wondering why no one wants to do business on my fair city despite everything being fine.
Then I realized that MAYBE I wasn't producing goods, the kind of goods produced by generic industry, AND (again, maybe) there were just too many of them for a 20k+ pop.
I had forestry, farming, and oil industry aplenty but no generic industry to produce goods. MAYBE the shopping malls and the other commercial buildings want to source locally and less competition to maximize profits.
With that in mind, I made a new city. I made sure that I had the generic industry this time along with forestry and farming. I zoned commercial bit by bit to make sure I have no more than 10 establishments.
Played the game normally - expanding the residential areas, making sure that the citizens are happy, pretty much everything mayoral. After a few weeks, there was still demand for commercial!
Expanded the city and there was still demand for it, albeit quite low. Then it was gone. I thought, surely because I had a couple of big shopping malls and whatnot, the zero commercial demand was understandable.
A few weeks pass with no demand for it and then one day a citizen tweets chirps me saying that local retailers need more goods! I thought maybe someone was just trolling me but I decided to make a new zone for general industry, after all, I haven't expanded my first general industry zone. So I did and holy poopy he was right! I had commercial demand once again! I also noticed that it increased as residents and generic industry came.
Made a lot of sense. Source locally + more buyers = high profits
More testing needs to be done but for now, I'm very sleepy and will just continue this the next day.