People are assuming they will want to stay below the 2nd apron (134% of the cap) which this year is about 179 million. After adding salary in the Smart/KP trade its basically impossible to stay below that number and bring back Grant as the team is currently constructed.
Now of course its debatable whether they will or should stay below the 2nd apron. The new CBA has some pretty steep team building penalties for being over that apron, but most of those don't kick in this coming year so there's no reason other than tax money why they can't spend over that limit this year. Buuuut those penalties will kick in the year after that, so it may very well be better not to sign up more long term salary like grant if you plan on paying brown/tatum/KP. If you do re-sign Grant there's a decent chance you just have to dump him in a year to get below the 2nd apron.
But right now it just so hard to know what they plan. Maybe they move Brogdon and open up money, maybe they don't care about the 2nd apron, maybe they bring Grant back on a good contract and trade him in a year for a pick. Who knows.
I think the main reason people think he's gone is Joe didn't really play him last year and with three rotation bigs he seems to have even less of a role.