Either you've entirely slammed the spam unsubscribe issue or somethingCandidates for "something else":
o The members using affected services learned to handle it,
o or left your group,
o or changed services.
o The spam filters of the affected services were told "not spam" often enough that they learned not to put your group's messages into the members' spam folders.
The first and last of which were the hoped-for goals of unsubscription as a teaching tool.