Sixers right now are not in rush in trading Simmons so they won't accept lower trade request offers.
If they are in rush, they have already settled but they are still waiting for a much good offer. I read an article before that if Sixers really wants to get rid of Simmons, they should accept even a role player in exchange. The problem here is, Sixers are considering Simmons as a superstar but other teams don' think that way.
The time is running and the more they wait for a good exchange, the more they are extending the team building new chemistry. Once the trade happened, their newly acquired players will have to work on chemistry first and we don't know how long it will get a good result.