Related to this, @dabrahams said this in an earlier discussion of this topic (in December 2015) (this is in response to a pitch for introducing FiniteSequence and StableSequence protocols):
First, we have tried not to create any distinct protocols with identical syntactic requirements, because we think it makes the world clearer; we think people are more likely to assign incorrect protocols when all the operations they want are available, but don’t have the right semantics. That isn’t to say we shouldn’t start doing it, but it would be a break from the past.
Higher protocol granularity has a high comprehensibility cost. Distinguishing protocols based on semantic requirements alone may make the library harder to understand. I’ve heard some people’s heads have exploded from simply encountering CollectionType.
Next, it’s a principle of generic programming that every protocol should be justified by both algorithms that exploit its requirements (e.g. extension methods) and some real-world models that can’t reasonably also conform to a more-refined protocol. For example, we have a ForwardIndexType because a singly-linked list has multipass forward traversal and can’t do bidirectional traversal. In order to evaluate any proposal for new protocols, we’d need to see all of these things.
(Edit: added the link.)