It's not a coincidence that Mathieu took part in two of the other longest matches in Roland-Garros : against Nadal in 2006 (very near to 5 hours according to my memories, and one match people should note when they speak of "Isner going from 5 sets against Nadal to losing to Mathieu what a shame"
) and against Canas in 2005.
It was also huge matches, esp. the one against Nadal, and the match against Canas was very sad/unlucky for Mathieu.
As somebody said, Mathieu is not "any" French player, he's brought a lot of emotions to people during all these years. He's also a very nice and sensitive guy and a huge fighter.
Although being French myself, I don't care much about other French players personally but Mathieu has always been special emotionally.