For the same reason they omitted the Prayer to St. Michael the Archangel.
I also thought there would be a spate of articles ,remarking on,and analyzing why the majority of the "helping" professionals,who lost ther lives in9/11,were Catholic.Now it could be that they were buried with Catholic funerals but were not practicing,it could be that ,it was the natural outcome of nepotism,or,it could be that information that was initially reported,was not true.It's conceivable that there was an overreporting of the number of police and firemen killed,or that the number of Catholics was not as high as initially reported. While I,unlike many, do not believe that all that happens on earth is God's will,I do know He could prevent it,if He thought it necessary,I do think He expects us to use these trials to bring more of us back home to Himself.
As individuals I think many,many people did just that with their time,prayers,gifts etc.;as a Church,I think we did not use the gifts we really have to offer,wisely.Just my thoughts.