Okay kiddo, have you ever heard of something called "germs"? They are tiny things we can't see that can make us very sick. Some people have learned how to make these germs even stronger and use them like weapons to hurt other people.
That's why a lot of countries got together and made a rule called the Biological Weapons Convention. This rule says that nobody can make, have, or use these dangerous germs as weapons. It's like a promise that everyone agrees to follow so that nobody gets hurt.
Just like we don't hit or hurt people because it's not nice, countries agree not to use biological weapons because it's dangerous and can hurt a lot of people.