Chaos was popular and became about as well-known as Space Marines to casual audiences (think of how many kids online you've heard type or say "BLUD FOR LE BLOOD GOD XD"), so they focused more on Chaos. It was profitable.
They also got a big staff change around the time Warhammer went to shit, 2008-2010, and many of the new staff were apparently avid Chaos fans.
There's also some level of fan hating in there, because even in the Storm of Chaos some fans complained that GW retconned the results to be more favorable to Chaos and kept shoving Archaon into everything.
Now, Chaos should be badass, strong, and scary, but some fans didn't like its new approach, even though it lost (an actual defeat and not just a "lol were just bored" loss like the shit NuHammer pulls).
GW responded to this by retconning it all and making Chaos stronger and Archaon more integral to the story; when fans complained more GW responded by retconning MORE lore and making Chaos even MORE strong and Archaon MORE integral to the story.
GW responded and still responds to nearly every complaint by making things worse. The more fans complained about Chaos being overpowered, the more overpowered they became. It happened in 40k first, but never picked up speed until Warhammer Fantasy was changed into Warhammer: The End Times (and later Age of Sigmar). when the Chaoswankery went to new levels.
40k players back then often said GW would never do such a thing to 40k and GW even said they wouldn't Age of Sigmar 40k back in 2015 or 16.
Now that Chaos is set to win unstoppably, the Emperor's getting some dumb storyline, and cartoony characters and casualized game mechanics are coming in, exactly like The End Times, it's looking like they lied, and we just didn't listen.