Mainly because Smart is an RFA and will be making less than Bradley next year. The luxury tax is a very real thing, and $20+ million for AB vs. $13 or so million for Smart is a huge difference.
This is, of course, largely conjecture. Someone gave Tim Hardaway Jr. a $20 million per annum offer.
As for that $300k, 1. It wasn't Danny's decision to make Hayward take less money. He offered him the max, now he needs to make moves like he no doubt said he would in order to give it to Hayward.
We don't really know what he offered him. I just refuse to believe that in a world where people actually talk to each other gutting the roster over $300k could happen.
2. It wasn't just Rozier in that case. It was Rozier+Yabusele (for next year, we'd presumably have him long term no matter what)+Morris. 3. The $300k assumes that Yabusele was willing to stay stashed another year - we couldn't clear his hold without his cooperation unless we traded away his rights. If he wasn't willing to sign the paper saying he wouldn't be coming over this season, we'd have been ~$2.5 million short, not $300k
You'd have room for some combination of these by trading Jae Crowder as well. Maybe he could have fetched you a real big so that we wouldn't have to be subjected to watching Morris and Horford trying to put a finger on the ball. We'll never know, I guess, because the brass is "excited about combinations" /cue obligatory facepalm+eyeroll here.
So if you want to keep asking how AB for Morris makes sense and ignore everything besides "AB>Morris", be my guest. But the other factors involved in the deal are why it happened. The question isn't AB vs. Morris, it's Morris vs. what else we could have done to clear the space. And clearly Ainge felt this was the best option
We moved Bradley, the player with the least value of the expendables, in order to put together an even more redundant roster. I'm sure there were "factors", but to me it seems the biggest one was falling in love with Crowder's contract. Doesn't strike me as good management, but what do I know.