Ray Raphael answered this question in his essay, 'America's Worst Winter Ever':

"The answer, in a nutshell, is that Valley Forge better fits the triumphal story of the Revolution passed down from generation to generation, while Morristown is viewed as an embarrassment. At Valley Forge, the story goes, soldiers suffered quietly and patiently. They remained true to their leader. At Morristown, on the other hand, they threatened to mutiny." (http://www.historynet.com/americas-worst-winter-ever.htm)

It has also been speculated by some historians that Washington's pleas to improve the conditions of his soldiers at Valley Forge were exaggerated to get as many supplies as possible. It was more difficult to bring supplies to Morristown, so a happy ending that winter was less in sight. Washington also did not want to get the news to either Congress or the British that his soldiers were more than on the verge of mutiny - on May 25, the Connecticut line actually did snap. A shipment of supplies two days later cooled down many tempers.

Another point is that more soldiers died at Valley Forge (around 1,000-3,000) than at Morristown (only around 100). Valley Forge's deaths and suffering were largely due to inexperience to a soldier's life. Though Morristown was worse in terms of weather, soldiers were more 'hardened up' by the time that winter came around. (http://www.nps.gov/morr/faqs.htm)