When you don’t know answer for some question – the answer is always money.
With the Viking Hats people knew that they would be available after the contest so they didn’t spend the gems/money and they waited to buy them with black pearls.
In the case of NFL hats people had info that they would be available only through Tower of misFortune, so they had two choices:
1) play, spend a lot of gems/money_on_gems to win all NFL hats and get +1 bonus upon completion, followed by buying more gems to play this “pay to win” game
2) don’t play
To sum up, it is just low-leveled marketing plan, encouraging you to spend gems in the chase of perfect Flock Power.