The core reason is the USA's addiction to foreign oil.
Shit happens. In any complex system, accidents and breakdowns will occur.
And it happens in pipelines, too. They just aren't reported as much in the oil-industry controlled American press--because of politics.
Try googling "Canadian pipeline spills"...
http://www.ctvnews.ca/business/pipeline-company-faces-greater-scrutiny-after-recent-spills-in-alberta-1.1354170
http://www.huffingtonpost.com/2013/07/05/crow-reservation-gasoline-spill_n_3550728.html
http://online.wsj.com/article/SB10001424127887323683504578563450142548548.html
