Book and page references are super important, especially when asking about prior editions.
At any rate.. finally found the issue you're talking about. The former errata lead had
this to say, which doesn't answer all your questions.
Personally, I'm skeptical about allowing armor mods to contribute to drain resistance pools. For game balance reasons. The very, very, very last thing Shadowrun needs is mages being even more powerful, and getting to stack armor mods onto your drain resistance pool does exactly that.
As for in-universe explanations go... other than indirect combat spells your armor is ignored. Magic affects you from inside out. Stands to reason drain would be, too. Ergo, gear that's not a part of you shouldn't help resist drain. IMO.