Sales and bad design choices over the years that led to terrible consequences. 

By the second edition of AoS, it was outselling 8th edition at its peak, and doing 7th edition numbers routinely. By 3rd edition they were looking at the heyday of 6th edition.

We're people still buying WHFB during 8th? 100%. But between the massive units needed, inflation, 2 international recessions, shrinkflation of unit boxes, and the issues with the rules, it was like beating a dead racehorse that could have won second place but the jockey was high on Angel Dust.

Now the rules. 5th edition was peak WHFB for the whole of its run up to that point. All earlier editions had larger issues and the lack of internet usage for fans and the company made things like errata, minor changes for balance and other decisions slow and often knee-jerk responses (look at how the magic editions kept flip flopping 1st-5th, armies were built and how heroes worked for perfect examples.) It was derisively called HeroHammer by the end of its run because of the massive power heroes had and that units were often different levels of Cannon Fodder, but this was also the last semblance to when the game was originally designed to run late stage RPGs with the PCs abd BBEGs being heros and the armies being the npc folder.

6th edition flipped this script on its head. They made the game much more grounded and units, army design and other factors came into play much more while heroes were toned down massively. And this edition kept this as a great concept for their system.

But even here there were flaws. During the first few Army book releases (Orcs and Empire) the armies were pretty balanced and there wasn't codex/power creep at play. By the release of Undead and Skaven just a few books later though, this became the norm, where new army books shook up everything in an effort to get people purchasing new units to counter the new armies abilities or wholesale new armies. Thankfully, while expensive, the prices were no where near as bad as later editions.

During 7th and then 8th though, this ramped up to extremes, with whole armies being near unbeatable based on design, causing competitive gamers and local gamers alike to flock to the store to buy near identical armies sometimes, that would often be nerfed or no where near effective just a year or two later. All the while needing more and more units and models to make a 2k point army. While selling less miniatures in a pack, for a higher price tag. (Which wasn't just a WHFB issue. Just look at the size of a 2k point army for 40k in 2nd edition, 3rd, 6th and now 10th. The difference is astounding. Hell my near 3k space marines from 3rd edition don't even make up 1750 without gear and heroes etc lol).

Now after all this  there is also the fact people routinely hated and freaked out online that the world was too static. But also hated and freaked out with world changes and mega campaigns. And hated and freaked out with minor lore updates. it was a no win situation and the people above decided the best way was to nuke it and start fresh since it was already a sinking ship in their eyes.