i got city lights on a planet once aswell, after placing tons of stars then placing planets orbiting them, the stars were light years from reach other, well, most.
the system was a sun-like star, but a giant blue star several light years away looked like the sun from Saturn or Jupiter, i THINK it was still lighting the planet along with its home star, but i dont remember for sure, as it was a couple months ago.
i placed it in the habitable zone aswell but its temp was like over 20 C.
so i would assume a basic requirements for spawning city lights is as follows:
1. A sun-like star (within .17 sun mass as the star was 1.17 sun mass)
2. Backround stars. (not 100% of this)
3. Placed in the habitable zone when created.
4. (possibly) other planets in the system, i created other planets orbiting this star before i placed the planet.
even then i think theres only a chance of it having city lights as it might require the planet to be sort of Earth-like in terms of atmosphere and mass, as i placed several other planets in the same orbit and they didnt gain city lights.
not sure if this matters but i think city lights are decided at placing the planet, so anything after that wont create city lights.
It'd help alot if the developers posted some info of how the game does this.