React's lifecycle methods and other similar features require some kind of "object instance" to exist. Since React was created prior to ES6, the React team created their own "inheritance"-ish approach, which is something that every JS library out there was doing as well. One of the main arguments for classes being included in the ES6 standard is that it's a common approach that libraries can converge on and use together, rather than every library inventing their own version of inheritance.
Since ES6+ is now the actual Javascript language standard, it makes sense for React to move away from their homegrown class definition approach, and build on top of the approach that is now standardized in the language. The React community has already eagerly adopted ES6, and while React.createClass() isn't going to be deleted any time soon, it's time to start encouraging people to move away from using it.
Either way, though, React's API _does_ depend on having something "class"-like available.