It's honestly pretty simple. It's GWs biggest media project to date, and they setup a year to work out the details. Anyone with half a brain would use the allotted time to work out the details. They aren't even working on a script yet, simply the general overall concept of what the show would even be and people were expecting Henry Cavill to have been in a trailer or something for it already. Considering they'll likely be using up the rest of the time in 2024 for negotiations, and then need additional time to actually write a script along with the rest of pre-production, then film the thing and post production, this show isn't going to be done till at least 2026.
Amazon has to write, cast, and film things the same way Disney does. The star wars crap they've been streaming has been getting announced 2-3 years before the shit airs, and that's because it takes time. Netflix? Same thing, it took 6 years from the announcement of the live action one piece project, to actually get the show(yeah, 2017 was when it was announced), Cowboy bebop's live action was also announced in 2017. Shows like the witcher, and amazons rings of power? Announced in 2017, didn't even have producers till 2018, and still took 2 years between seasons 1 and 2.(it's also still shit but that's beside the point). These shows are not filmed in a way where they can crap out an episode every 2 weeks on a sound stage like the adventures of hercules or star trek tng.
WHFB was already dead by the time the first total war: warhammer game came out. Additionally, since it isn't like GW develops the shit themselves they don't have much say if it's a quality game or not because they simply license their properties out to anyone willing to pay up. That's why AoS gets shit video games like Realms of Ruin and Stormground.
It's arguable that actually not skipping editions for faction rules in WHFB might have helped, but the same thing has happened multiple times over the history of 40k without being an issue. WHFB wasn't selling for shit by the time they made the decision to kill it, and there's been former GW employees stating as much in interviews. Even dumber is that when WHFB died, and even when "the old world" got announced, was there any uptick in people playing WHFB? No. There's a reason the rumor of the 40k tactical marine kit outselling the entire WHFB range by itself isn't unreasonable.
