Better receivers. Worse offensive line. New system. Maybe very limited time to actually work with his guys. Plus he's a year older.
Last four years:
2016 (age 39): 67.4%, 8.2 y/a, 6.5 TD%, 0.5 INT%, 112.2 rating
2017 (age 40): 66.3%, 7.9 y/a, 5.5 TD%, 1.4 INT%, 102.8 rating
2018 (age 41): 65.8%, 7.6 y/a, 5.1 TD%, 1.9 INT%, 97.7 rating
2019 (age 42): 60.8%, 6.6 y/a, 3.9 TD%, 1.3 INT%, 88.0 rating
There's still nobody I'd rather have as my QB for one game to win it all. But he's definitely no longer the best QB in the league, and his numbers and the trend above just aren't hard to see. Less talent? Sure, and that impacts his numbers. But he's going to be 43 with this trend. It wouldn't surprise me at ALL for him to put up worse numbers in 2020 than he did in 2019.
Then again, it's Tom F-ing Brady, so maybe he goes OFF this coming year.