Because they are not making the playoffs. Their goaltending sucks, Vlasic is terrible, Burns and Karlsson having not a good season, other 3 D are UFA. Labanc needs a new contract unless they want to trade him. Long term their core is Couture, Kane, Meier, Labanc. They need the depth above all. To replace Thornton and Marleau they will need picks. They traded their 2020 1st rounder to Ottawa. Reality is they are the prey to be taken by the vultures. Bruins have the assets, the need and the trading history to take the best chunk of their current situation, not the carcass. Makes just too much sense for both sides to move Hertl.
I think a package around 2nd*+ Heinen + Senyshyn + Zboril, *conditional 1st if Bruins go to finals.