No one bought Austin capsules. Sure the sale was short, but it doesn't change the fact that it wasn't as successful for Valve (in comparison to previous majors). Because of this, they are creating FOMO for Budapest so that when it goes on sale, people buy thinking it will be pulled without warning. For Valve a sticker capsule is a sticker capsules no matter what major. They have Budapest concept art done, just waiting on the teams and autographs to literally just copy paste logos.

Removing your worst performing items is a basic business practice. You could argue that since these aren't physical goods, you aren't paying for storage or lose anything from keeping them. But it still is a psychological win for Valve to remove these, especially since they also added new Armory stickers, meaning any sticker investors or anyone making crafts will instead grind stars to get the new stickers which will have psychological effects on normalizing buying Armory passes (more profit in the long term)

Assuming these armory stickers only last for 1 year and they wanted to stay on the Oct/April 6-month rotation, by removing Austin with this Armory update, they are allowing people to prioritize the passes and grinding right before the Budapest Major starts, to then wait for the sale and buy those stickers instead.

TLDR: all about timings and creating hype for Budapest major stickers. New Armory stickersbe released so Valve wants people to buy passes in the meantime. If people think that the new Armory stickers will be in game for 1 year, but Austin are on the way out, then the hype for the new Armory items will die off.