The answer lies in one of the show's most engaging aspects: its frequent flashbacks to the 1970s, exploring the early years of marriage and parenthood for Jack (Milo Ventimiglia) and Rebecca (Mandy Moore).
In order for a series to be eligible for the Contemporary Costumes nomination, at least 51 percent of the action has to take place in the past 25 years. The episode submitted for "This Is Us", Season 1 finale "Moonshadow," has a majority of scenes taking place in the 1970s.
A TV Academy representative tells The Hollywood Reporter, "Having reviewed the 'Moonshadow' episode of 'This Is Us,' the Awards Committee noted that the entrant unfortunately miscalculated the proportion of scenes that were period versus contemporary. In fact, the episode is predominately period. The entry (and subsequent nomination) are thereby disqualified."