Helm Classic was introduced at the first KubeCon... Wayyy back in November 2015. Kubernetes 1.1.1 was released earlier that month, and 1.0.0 shipped only 4 months prior to that in July.
Back then, Kubernetes had no concept of a ConfigMap. ReplicationControllers were all the hype (remember those?). The Kubernetes API was changing rapidly. When Helm 2 was being built, we needed an abstraction layer from the Kubernetes API to allow ourselves some room to guarantee backwards compatibility. Tiller was created as that abstraction layer. It provided us with a layer where we could control the input (via gRPC), the output (also via gRPC), and provide some backwards compatibility guarantees to users. We're pretty proud of the fact that Helm has maintained a strong commitment to backwards compatibility since 2.0.0.
Over time, Kubernetes' API layer has become more stable. Helm 3 is our opportunity to refactor out some of those protective layers we put in 4 years ago. Tiller being one of them.
Hope this helps provide some context.