The reason for this is actually because of the addition of high-poly fan-made cosmetics and hats. Valve didn't want to degrade the performance when community-made cosmetics were added (which were usually MUCH more intensive and detailed then things added by Valve, due to them not putting a poly-count limit). They wanted the game to be runnable by the systems that always could run it, but they also had the process of supplying content through users, so they had to change some of the graphics to keep performance up.