Why? It is abundantly clear that monopolizing the entire screen to the detriment of the specific product in question leads to the worst possible outcome for almost everyone involved. Viewers hate it. Streamers hate it. The only people who like it are the shortsighted executives at both Twitch and their ad partners and only because they refuse to innovate at all. Hilariously, streamers and their ad partners came up with better streaming advertisement options completely independently and were so effective that these new rules are aimed squarely at eliminating them. Instead of understanding WHY this was the case and capitalizing on it (Twitch can provide streamers pre-defined overlays with ads that run continuously from Twitch ad partners and the streamers get a cut of the ad revenue) they ratcheted down the screws on the better way in favor of the their antiquated, outmatched, and maligned system.
Twitch is fucking dead. No one likes watching streams with forced ads that appear randomly and impact the gameplay viewing experience, so they were already on the downturn, but hopefully these new rules nail the coffin shut much faster so we can move past their rampant, incurable stupidity and on to the next thing.