Well, this isn't entirely true. I have copied a link at the very bottom of the three links below that will take you to information regarding how this novel was received when it was first released. You will be able to read there about the ways in which this novel was viewed by the critics. In particular, you might be interested by the somewhat negative reviews made by critics who saw the non-traditional elements of this novel as being "gimmicky."
However, for me, this is certainly one element that helps make this novel into a cult classic, as it deliberately breaks free or challenges the traditional form of the novel by having interspersed and carefully placed into the narrative various pictures, blank pages, pages from a notebook with questions on them and famously a flip-book showing a person jumping from the World Trade Centre. Arguably, what makes this so significant is the way that the author uses this supposedly "gimmicky" features and weaves them into the plot of the story to make a very moving tale come alive.
In addition, I think its publication in the years after 9/11 also helped cement its popularity as it managed to create a hopeful lament from a very personal standpoint on the tragedy of 9/11. This way of reinterpreting what happened on that fateful day clearly shows the author's attempt to resonate with the grief and capture something of the despair of the US in his work. The fact that he is able to go beyond that and offer us hope shows his talent and skill.