A ranking system needs to look at a lot more then simply the number of competitive lists an army can build, you also need to look at the relative strength of those lists vs other competitive lists. Even if a codex can field 100 competitive lists, but none of them have a great chance against only 2-3 other lists thats a big blow to their overall competitiveness. The relative weaknesses of the lists you can field is, in my opinion, much more important then its strengths.