The answer is 'Because President Barack Hussain Obama, decided to continue the War in Afghanistan'
By the end of the 8 years of the Bush era, the Afghan war had dragged on with not a lot of results. It was increasingly becoming unpopular and while Obama had a chance to end the war. It never happened.
More and more Americans and Afghans died with a political stalemate the biggest achievement.
That is why the cartoon describes it as Obama's Vietnam, referring to the now considered, pointless war in Vietnam that cost a lot of lives and money but achieved nothing.