I understand GW froze the storyline/timeline after the 13th crusade event in 2003, and then did nothing with the "main" storyline until the Roboute / Yncarne story from 2017. Did GW ever tell why everything froze? Anyone having an idea what happened?
From my perspective as someone that doesn't take much notice of the fluff, or read any of the Black Library books I have never considered that there was much of a 'main' storyline. I have always considered the bits I have read in the Codecies and rules books to be nothing more than a background. That is, just something to give your games a bit of a setting, or narrative as GW likes to say.
I think it is clear from how some of the characters like Ghazghkull and Tycho were introduced into the background from Battle Reports that GW never had, or intended to have, a structured and advancing timeline.
So I don't think that the timeline 'froze' it was just never meant to adavance in the first place.
I think 'competition' from Mantic, Corvus and the like has possibly forced GW into rethinking how to keep their customers engaged.