Now that I think about it, should Twilight become a goddess of sorts, she won't have many worshippers, or see them for long.

Think about it. While the sun is up, many would usually be up and awake for most of the sun's duration in the sky. When the moon is up, most would spend their hours asleep between 6-8 hours. While people are usually awake during the twilight hours, twilight only lasts for about 30 minutes; more so if you're up in the poles, assuming Equestria shares a similar tilt to Earth.

Besides, Luna would be upset with the idea of having somepony taking 30 minutes out of her time, every sunrise and sunset. Celestia gets too much attention as it is. Imagine the politics!