Why did Tom Boonen not participate in Tour de France in 2008?

Explanation

Tom Boonen (born 1980) is a Belgian professional cyclist. He is a sprinter and a specialist of the spring classics. He has won Paris-Roubaix three times -in 2005, 2008 and 2009. He was also World Champion in 2005 and won the green jersey in Tour de France in 2007. Boonen started his career with US Postal in 2002, and changed to Quick Step the year after. In June 2008 he tested positive for the use of cocaine, which cost him the participation in Tour de France the same year.