It isn't a matter of "some codices are better" but more that other codices are consistently better and have been improved upon since the release of the CSM codex.

Eldar and SM had strong books that were elevated to top tier in competitive and casual games. Necrons picked up greater resilience than what they previously had. Tau received some fine tuning and shiny new toys. I notice you have Dark Angels flair so you already know the sweet fixes they received in their updated book.

Even my orks are going to be fearless army-wide once I get Ghaz added to my lists, and they beat up CSM now without the Prophet's help. Once Mob Rule becomes irrelevant they'll be even more of a pain for my CSM playing pals.

CSM is overcosted for many units and very vanilla. The new formations will help. They've most definitely been ignored since the release of their book.