Adam Duvall's contract makes him highly desirable as a trade target
Lets get the easy one out of the way: teams buying on the trade market this time of year love guys whose contracts are expiring at the end of the season. Such players are cheaper to acquire in terms of prospects and keep teams from having to be saddled with long-term financial commitments that they could regret down the line.
Duvall's contract is firmly in that camp as he signed a one year, $7 million deal with Boston before the 2023 season. Any contending team that is looking for an outfield bat wouldn't have their future payroll plans upended to acquire him and finding another $3-3.5 million in 2023 dollars at the deadline isn't particularly difficult for most squads.
When you compare Duvall's deal with the other guys on the outfield market, he holds some clear advantages. Most of the other guys that could be considered rentals either aren't going to be available (Conforto, Bellinger) or haven't been very good. There are some interesting bats with some team control that may be available especially with the Cardinals deciding to sell, but they all have some warts and will be significantly more expensive to trade for given that team control.
For a lot of contenders, they aren't looking for a long-term fix to a roster problem at the trade deadline as free agency will give them more/better options. This is about making it through the end of the season and Duvall is one of those bats that can give a lineup a boost without mortgaging a team's future.