Why did the England football team not fulfill their potential at the World Cup?

Source: http://www.bbc.co.uk/iplayer/episode/m002wbmg?at_mid=fwcDQ6qW9A&at_campaign=England_2006_The_Golden_Generation&at_medium=display_ad&at_campaign_type=owned&at_nation=NET&at_audience_id=SS&at_product=iplayer&at_brand=m002wbmg&at_ptr_name=bbc&at_ptr_type=media&at_format=image&at_objective=consumption&at_link_title=England_2006_The_Golden_Generation&at_bbc_team=BBC