So, it seems, the secret to having popular concerts in Cities:Skylines is: High Ticket Prices.
I suppose the reasoning is something along the lines of: “Wow, these tickets are expensive! NESTOR must be awesome!” But I don’t know.
The other thing I discovered is that adjusting settings doesn’t have an immediate effect, because a band’s popularity doesn’t change until right after a concert ends. That makes sense.
The Music Center festival area hosted 18 concerts, had one sold-old show, and a band gained a popularity of 85. Moonlight Fields now has a level 3 festival area, with an audience capacity of 1,000.