Hello,
Disclaimer: Just a friendly note; My answer is my opinion because there was not much information. Anyway hope this helps you.
I believe death is a very popular theme because readers are afraid of it. So when a main character dies or someone in the story died it brings a sense of drama in the story and sadness, It will also what the reader to keep reading to see what the other characters reactions are. If there was no death in a story or no conflict most stories would not be different and mostly boring.
Have a great day.