It's pretty simple, Tatum initiates the offense, Brown can't. Brown has to rely on the offense running through Tatum or Kemba (or last year, Hayward), he simply doesn't have the vision or handle to do what Tatum does.
I love Brown and what he's doing right now, but every opponent we play builds its defensive game plan around Tatum, Brown was an afterthought. But as hot as he's been, we can expect that to change going forward.
Plus, this is a 7 game sample. As great as he's been, 7 games does not signify this is who he is from now on. Tatum had a 7 game stretch last year where he averaged 34/9, shot 50% from 3, and was NBA player of the month based on that stretch. Is that who he is? The NBA's best player? No, he's a great player who had a phenomenal run. Is this 7 game stretch who Jaylen Brown is? A 60% shooter and top 5 scorer in the league? Very likely not. No one wants to admit it, but history shows us that whether it's MJ, Lebron, Kobe, even the best scorers shooting % will eventually come back to earth.