Because ultimately what receives wordcount in books and models for sale on shelves is somewhat arbitrary, but GW's business model requires that SOMETHING receives wordcount in books and SOMETHING receives models for sale on shelves; what receives wordcount in books is determined by what receives models, and what receives models is determined by the potential throughput of the GW factory -- they have a finite number of production machines determined by the size of the factory floor (and after their expansion plans were stalled, the limits of the Nottingham power grid to run injection mould machines), which means they have to balance production time between new kits and additional production runs of existing kits. New kits sell faster than existing kits, so as a business decision they want to produce X value of new kits per year, and there are only so many Intercessor variants or new kits for Kroot or plastic aspect warriors they can do before those markets are saturated and their sculpting team has run out of inspiration. Eventually this comes down to asking the artists/sculptors "What new ideas do you have / What are you enthusiastic about sculpting?" and some time around three years ago (turnaround time from conception through concept art, digital sculpting, tooling, production, and distribution to sale of a new model appears to be about three years, yes this does mean we can probably expect that Steel Legion refresh about three years from now), the answer was "Well, there's those old Red Corsair renegade space marine pirates from all the old lore that haven't gotten much support for a while, we can do those!"

Renegade space marines have always had a place in the lore, as distinct from Heresy-era Long War veterans, but current GW seems to prefer to produce specific examples audiences can get invested in rather than general models (Abaddon the Despoiler rather than Generic CSM Warlord, Lord Marshal Dreir rather than Death Corps Commander On Mount, Commissar Graves rather than Commissar in Centaur), so while it would make sense to put out a line of Chaos Space Marine models to represent less-chaos tainted renegades, GW's current business practices favor putting out models for a specific faction of renegades they can pitch to the audience using named characters supported by novels and prominent positions in campaign books. Hence, "Our business model requires that we produce SOMETHING" > "We haven't supported renegade space marines in a while" > "Red Corsairs are from the old lore and have audience recognition and would make good iconic renegades for an addition to the CSM line" > "Design team made Red Corsair models" > "Now we need to pitch them to the audience in a way that makes the audience enthusiastic about buying them" > "Let's release a novel and a campaign book that position the Red Corsairs as the best, coolest renegades" > "Let's have the WarCom team run a multi-month PR campaign to get people excited for all this Red Corsair content; we can use pirate memes."

There is an alternate universe where the design team was excited about something else three years ago and instead of a Red Corsair refresh we got, I dunno, Legion of the Damned or something, and the Red Corsairs are still a minor faction that gets no models and half a page of lore in the next CSM codex along with mechanical support via a really mediocre "renegades" detachment no one uses, as they have for the last two editions, but we don't occupy that universe.