Hi! In this case we don't use the -n because there is the verb "esti" in the sentence. It's a verb of state, it describes something, without any action. Nothing make an action, nothing is subjected to an action.
It's the same thing with 3 other verbs: Resti / Stay Aspekti / look like Ŝajni / seem
I hope that it will help you! Sorry if I made mistakes, I explain something in English, for Esperanto, and I'm French ^^ So, I had to use a translator for some words, if someone want to correct anything, go on ;)
Have a nice day!