To be honest I don't know what was the motivation of TC39 for adding class syntax to ECMAScript. Perhaps for easier transition of object oriented people (Java/C++) into JavaScript?
Yeah this was it. People coming from more traditional OOP languages were confused by prototypes and the fact that there seemed to be different ways to do OO in JS, so they added some syntactic sugar to make it easier for people moving to JS. They saw familiar looking class structures and it made them feel more comfortable.