Good question! It's because there's an aspect of conversations that differs between different models: the way the previous messages are injected into the context of the prompt.

I went back and forth on a bunch of different designs, but eventually decided to try to make it so that each plugin that implemented a new model would only have to subclass Model and add new methods.

You can see all of the design arguments I had with myself about this here, across the course of 129 pull requests comments: https://github.com/simonw/llm/pull/65