Answer
Renderers in Xamarin.Forms were tightly coupled to the control hierarchy and carried significant overhead because each control inherited a full renderer. Handlers are loosely coupled through interfaces (IView/IElement), have a smaller surface area, are easier to unit test, and allow customization via mappers without inheritance. This improves startup time and reduces memory usage.
Real-world example Profiling a migrated app shows faster page load because handlers instantiate less object graph per control than the old renderer pipeline.