Perhaps the next-best alternatives to fill the same space aren't paying as much. Perhaps Google was strategically overpaying AdSense partners for a while, to achieve critical mass (of advertisers and publishers) and forestall the development of competitive networks.
Now, having achieved an enduring dominance in this two-sided returns-to-scale market, they're dialing back their AdSense payments. Only the emergence of a strong alternative would prompt them to continue paying the same premium that was offered during the capture phase.
(And even if Google thought a competitor of similar scale was likely to arrive – eg Facebook – they might want to continue charging a lot for their market-making, building a large warchest, until the competitor arrives. Then, crush the upstart by dynamically charging very little, making AdSense extra-attractive for exactly those advertisers/publishers of greatest importance against the upstart. Similar to how a hundred years ago, Standard Oil was accused of using targeted rate changes to undermine upstart competitors.)
Maybe ad-blindness (if not outright ad-blocking) is spreading more widely among the masses, especially in AdSense scenarios. Maybe AdSense was only ever meant as a moat to protect AdWords from possible network-of-ads competitors.