According to NHL stats found here, the number of total power play opportunities went from:
- 14,390 during the 2005-2006 season to
- 10,227 during the 2008-2009 season to
- 8,055 during the 2013-2014 season.
In 2005-2006, the lowest number of power play opportunities by a team belonged to the Florida Panthers with 411(1), and in 2008-2009, the highest number of power play opportunities by a team belonged to the Montreal Canadiens and the Carolina Hurricanes with 374 each(2).
In 2008-2009, the lowest number of power play opportunities by a team belonged to the New Jersey Devils with 307(2), and in 2013-2014, the highest number of power play opportunities by a team belonged to the Philadelphia Flyers with 294(3).
Why have the number of power play opportunities steadily decreased since the 2005-2006 NHL season? Excluding the lockout-shortened 2012-2013 season, there has not been one increase in the number of power play opportunities. Have there been changes, such as rule, officiating, or playing style changes, that objectively explain this phenomenon? Or are other factors in play?