Reason #1: The Miami Heat can make the deal easier for the other teams involved
The Blazers don’t want contracts that would hinder their potential rebuild, so players like Tyler Herro and Simmons don’t fit into those plans. If the Nets are interested in Tyler Herro (as they probably should be), then the Heat taking Simmons back along with Lillard would simplify things and remove the potential complications of bringing in a fourth team.
If no fourth team is willing to take Simmons on (which is highly likely) then a trade involving the Heat, Blazers, and Nets is very unlikely to happen. If the Heat are desperate to acquire Lillard and go all-in, taking on the risk of acquiring Simmons and his contract may be a necessary sacrifice to make.
With not much else going on regarding rumors of other third teams getting involved, the Blazers will likely look to other teams with better packages for Lillard. If the Heat want to make this specific framework happen, they may not have a choice but to take Simmons.
The Heat taking Simmons would make the other teams happy, but that would also mean they’d be taking on $83.5 million in salary between two players. However, the Heat have the salaries to give up to make this trade happen.