Like others have already alluded to, Ainge was never going to trade down if he couldn't get the deal with Philly done, i'd say that 76ers were afraid of the Lakers taking Fultz.
Here is a quote from Ainge after the 17 draft:
"The draft was very even we felt at the top, all the way through maybe five or six," Ainge said. "And it was very difficult. There was a lot of players we liked in this year's draft."...Furthermore, Ainge added that Tatum was who they would have taken with the top pick if they had kept it instead of making the deal with Philadelphia. "Yes, we would have picked him with the first pick," Ainge said after the selection.