To keep him off of other teams. It is all well and good to not want to give up the farm for Leonard, but Boston can kiss any shot at winning the title over the next 3-5 years if Leonard ends up in LA with James or on the Sixers (not to mention the Warriors are still out there).
If only Leonard ends up on Philly, they're still not better than Boston. Even if he and James end up in L.A., that's not a team I'd consider unbeatable unless they get George too.
Philly is absolutely better than Boston if Leonard joins them if the trade is something like Covington, Fultz, and 10. They could even add Saric and 26 and they are still likely better, and in that scenario they still have cap room for another max player or can just re-sign all of their free agents i.e. Redick, Ilyasova, Belinelli, and Johnson.
This is a pretty darn good team
PG - Simmons, McConnell, Bayless
SG - Redick, Belinelli, Korkmaz
SF - Leonard, Anderson, Luwauw-Cabarrot
PF - Ilyasova, Johnson
C - Embiid, Holmes
That team is absolutely stacked