AFAIK the major driver for that was because PowerPlay 1.0 forced leadership's hand via some nonsensical quirks of how PP 1 was implemented in-game. AD's success would have been severely impaired without those leadership actions. Mystler referenced that in the reported issue referenced in the letter:
"Over the last 8 years, we have had to deal with quirks of the Powerplay Ethos not allowing Imperial factions to be favorable in Aisling space and thus having to sadly tell some eager PMFs that they are not helpful. We love that this is gone [with PP 2.0] [...]"