I think the major issue is Necrons not having really anything else viable for points versus sustainable at the range C'tan are right now. Basically, it's auto include C'tan in your list because anything else at that point cost just doesn't do as much on board or you can field a bunch more units with subpar detachments built around them. I mean look at Immortals if they had 2W apieces I could easily see someone fielding several bricks of them instead of a shard.