This question rests on a misconception, as Muslims have consistently and repeatedly denounced terrorism since September 11, 2001. For a large sampling of such condemnations, see this list. Another sampling is on our website.
Unfortunately, these statements are rarely noted in the mass media in the US, leading many people to think erroneously that Muslims have not denounced terrorism.
This question, however, could also be answered with another question: why should Muslims be expected to repeatedly condemn terrorism? Are Christians or Jews expected to denounce violence or every irresponsible or destructive statement or action made in the name of their religions? The question seems to assume that Muslims support or condone every act committed in the name of Islam unless they specifically state otherwise. This assumption is clearly unjust and unreasonable.