What are the main reasons why Ralph calls an assembly so late at night?

Ralph calls the meeting because he is frustrated and unsure of his position. Things aren't being done as planned. As a result, the meeting directly addresses his frustrations. He talks about the "beastie" and assures the boys there is no monster on the mountain. He admonishes their habits and tells them to use appropriate areas for relieving themselves. He also talks to the boys about their need for shelter, and the fact that everyone must chip in in order for their shelters to be constructed the proper way. Most importantly, Ralph addresses the signal fire and the need for the fire to burn continuously in order to signal passing ships or planes..... in order to be rescued.

An evening meeting was called because Ralph's concerns were immediate concerns. They'd never had a meeting at night. Unfortunately, the timing couldn't be helped. A ship had passed that day, the opportunity for rescue had been presented and lost because the signal fire had been left to go out.