Ventilation space should be 1 sq ft per 10 sq ft to assure adequate ventilation in coop. Make sure you have that. My guess is they just like it and got used to it.
Your spacing sounds fine. The height of a perch is a little high. They usually recommend not over 30 inches for a roosting bar as chickens are ground dwellers and this reduces risk of keel bone injuries. So the rest is just training them. I do keep a solar lantern hanging right outside the coop...
You could provide pictures for us to evaluate the coop. In general just make sure that they have enough space 4 sq ft per bird in the coop. Roosting bars that are easily accessible. 1o-12 inches of roosting space per bird. Make sure clean and ok. I put mine in when went below 30 degrees as their...