My take on why the 4 of them are not part of the tribute as a whole:
1. They all felt it was too soon to come together as a group, it would of been too emotional to get through. Similar to how Mike Campbell talks about not being ready to fire up the Heartbreakers without Tom being there. Yes, Fleetwood Mac operated without Christine for many years, but circumstances are different now.
2. 3 of the 4 agreed to do it with one member declining.
3. Irving is driving decisions behind the scenes, including who's being involved and who isn't. For a variety of reasons.
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