I think they thought PL team at home, there'll be massive demand. They neglected to consider kids being back in school combined with a late kick off, it's on TV, Forest are a PL team but not a big draw, they won't be playing their first choice players, still the early rounds of the competition, etc.
Hopefully they learn from this. I love the idea of including home cup games in season tickets, but can't see them going that far. Same prices as last two games should definitely be the plan next time.