Long term, the React team plans to investigate concepts like "stateful functional components", but until then, classes are the most straightforward way of having lifecycles. The existence of ES6 classes is based on the wide range of third-party "class" implementations across the JS ecosystem, so clearly there was a desire to have them available. Since they're available, React is using them.

That said, I tent to follow more functional patterns for workflows (prefer Redux and similar extension), but for some components the class syntax is usually nicer. Most components are probably best served as simple render methods though.