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They are setting up excuses ahead of time. From WashPost:

[...]It is also unclear whether he will reach his attendance goal of 50,000 people.
Some local groups have been slow to sign on to the prayer event. Rizwan Jaka, a spokesman for the All Dulles Area Muslim Society, one of the region's largest mosques, said the congregation was too busy with Ramadan and other activities to become involved.
[...]
Jihad Sileh, a spokesman for the Congressional Muslim Staffers Association, said his group's regularly scheduled midday prayer service is at 12:30 p.m., so it won't be an official part of the prayer event. The congressional association is also sponsoring a 1 p.m. prayer service for the Congressional Black Caucus at the Walter E. Washington Convention Center, so many staffers will not have time to head over to the Capitol for outdoor prayers.

The interesting part is how the Black Caucus has so many Muslim staffers.