Aren't Steelers fans supposed to be the "best" in the NFL?

Why were there only 45,873 fans in the stands Sunday? While it was still marked down as a "sellout" because no tickets were available, because the the NFL uses turnstile counts to get a more accurate count on how many people are actually in the stadium, they only counted 45,873 in the stands. The capacity of Heinz Field is 65,050, nearly 20,000 empty seats. That stadium would have been packed if they were doing better.