Over the time, I think I understood the following: Riot wants something the way they want.
If Riot wants x champion to be played in x role, Riot will nerf the champ in its current role (that's supposedly not the role it "was supposed" to be in) in order to make it play the role they want. Qiyana JG for example; they wanted her mid, so they made her bad jungle.
Riot didn't want Sanguine Blade to be a duelist item. Riot wanted it to be a splitpushing item.
That's why Riot decided they will remove it to add the item they want (that fulfill the sole purpose of splitpushing). Sanguine simply isn't what they wanted.
Not entirely wrong, though I think the phrasing here leaves some things that can be misconstrued. When we design things we always have a specific goal in mind. I can't speak specifically to Sanguine/Hullbreaker, but for example when we make a champion we have a goal lane that we want them to be good in, and maybe another lane that we are ok with them also being good in. We actively want that specific champion to be good somewhere, so it makes sense for us to make changes to make them better in that role (and sometimes, worse in other roles). We can make the same correlation for items. For example its pretty clear we want Luden's to be a mage item, so if it was good on Tanks (somehow) it would make sense for us to make changes to make it either worse on tanks, or better on mages to try to reach those goals. Things get much more complicated when you really dive into the nitty-gritty, but that is a very simplified explanation.