CodeSystem and ValueSet both have an identifier 0..*, where as ConceptMap is 0..1. THe identifier element serves 2 overlapping use cases - where an identifier is assigned to the concept at source, and it needs to be tracked around (that's canonical URL, except for systems that don't use URLs e.g. older value set registries like VSAC that are OID based). 

In addition, the identifier allows intermediaries and facades to networks of repositories to assign and track identifiers on resources as they are managed across multiple related networks.

There doesn't appear to be any functional reason why ConceptMap is different to ValueSet and CodeSystem in this regard