How come some kids don't get nervous when speaking in front of crowds but the rest of use do?

Extroverted. Some individuals are constitutionally extroverted and enjoy pubic speaking, being the center of attention at large parties, being a class president, cheer leader, politician etc. They may or may not have autonomic stability as part of their inheritance, if they do they would make great jet pilots and astronauts..."the right stuff", the book is all about the flyboys with nerves of steel.
Other way around. Most people don't have performance anxiety but those who do could be so for many reasons: shy, being unprepared, feeling ill etc. The response you have to a crowd would be from how you are conditioned to crowds and other people as a child. If your parents or other family didn't like being in public, you probably won't.