Because during his own career, Houdini always claimed to be an American -- born April 6, 1874 in Appleton, Wisconsin.* In fact, it's more likely Houdini symbolized for his audience a new breed of 20th Century American supermen. This was especially the case in Europe where Houdini was billed as "The Elusive American" and had posters that showed him wearing shorts made of the Stars and Stripes (and this before Rocky Balboa). An American superman was certainly how Houdini wanted to be seen.
So it must have come as a surprise to those who had known or had seen Houdini in life to learn that he wasn't an American after all. Many first learned the truth of Houdini's Hungarian birth 33 years after the magician's death with the publication of Houdini: The Man Who Walked Through Walls by William Lindsay Gresham in 1959. Some in the magic world still didn't accept it until 1972 when the S.A.M. National Council investigated the issue and published their conclusions in The Houdini Birth Research Committee's Report (above).
But the truth of Houdini's real birthplace and birth date was actually first uncovered and revealed in December 1932 in a German magic magazine, Die Magie. Patrick Culliton recently translated and reproduced that historic article on his website, Houdini's Ghost. It makes for interesting reading, especially the passage below in which the author talks about trying to publish this historic finding in an American magic publication and being rebuffed:
I believed that I was rendering our American friends a service and sent the original excerpt from the register together with the reproductions of the above-mentioned letters and pictures, to the S. A. M. [Society of American Magicians] for publication in SPHINX. To my great astonishment, I received from Mr. B. M. Ernst, President of the S.A.M. a cool letter of rejection with the statement that he could in no case accept that my conclusions were correct, and since it could only have been an error, he also advised me, in a tone that seemed disconcerting, against the publication of this material.
Here's another related thought. Before 1903, Houdini would list the year of his birth as 1873. This changed to 1874 with the new passport application he filed in Europe when he officially designated Appleton/America as his birthplace (Hardeen did the same thing). Why the change? Could it be that Houdini got a look at his birth certificate when he travel to Budapest during his European tour and discovered his real year of birth? If it was easily accessible in 1932, it would have been no problem for Houdini to get a copy in 1900. It must have come as a shock for Houdini to learn that he was actually a year younger than he always thought.
Below is copy of the birth record taken at the Registrar's office at the Pest Jewish Community on November 9, 1932 and referred to in the Die Magie article -- the document that revealed Houdini's greatest secret.