Interesting one, IMO. I am thinking about doing a nurgle CSM army, which is a bad idea given the current dex but thats not important here. I have noticed quite a few things that make me think GW hate nurgle especially, and chaos in general. Reasons:

1) All the other chaos gods have plastic lesser daemons. (Tzeentch only in second wave, but still.) What would cost you £15 for khorne costs £32 in nurgle.

2) All the other lesser daemon kits look really good. (Tzeentch ones dont look as cool as the metals, IMO, but they still look cool.) The nurgle ones have some elements of coolness, but nowhere near as nice as say, Daemonettes, or the khorne ones.

3) poor one, but fantasy 8th sort of nerfed plaguebearers, and every army is getting FNP now.

4) Plaguebearers, 7 for £20. I know 7 is the number of nurgle and all that, but khorne beserkers get 12 for the same price.

5) The august '10 daemon prince kits dont have anything in that scream nurgle. They do khorne, and to an extent slannesh and tzeentch.

Why? I struggle to think why. The only thing I came up with is:
Nurgle is an army that begs for GS work, and customisability. This attracts veterans and experienced hobbyists over kids, and they want to be selling to kids.

Am I being a bit funny? GW isnt supporting CSM in general, really. But I find it slightly worrying that they can get away with 3 lesser daemon kits and 2 metal boxes.