I think the answer is that with City, the narrative is almost entirely that they bought it. Yes, they’re a great side, but there was no struggle, no adversity overcome.
Like it or not, that United side had a large number of homegrown players, Ferguson was almost sacked at one point, missing key players in the final (Keane), and won through two dramatic late goals. Everybody will remember that game, and their treble. There’s a compelling story to be told.
With City, it’s a shrug and everybody moves on and forgets. It doesn’t feel like a monumental achievement, because it’s been inevitable.