I really wouldn't be surprised if they combined daemonhunters/witchhunters/etc. all into one codex. But as was mentioned. SM got their codex updated before all the others because they're the golden boys of GW. They rake in the money, they're the first army anybody usually plays as, and most of the fluff revolves around them. Next in the pipeline's the IG as far as I know. Then after that, who knows. I doubt my necrons will be getting an update anytime soon
"There is a cancer eating at the Imperium. With each decade it advances deeper, leaving drained, dead worlds in its wake. This horror, this abomination, has though and purpose which functions on an unimaginable, galactic scale and all we can do is try to stop the swarms of bio-engineered monsters it unleashes upon us by instinct. We have given the horror a name to salve our fears; we call it the Tyranid race, but if it is aware of us at all, it must know us only as Prey."