WHO says 42 days. A male survivor can carry viable virus in semen for 90 days. US Army says Ebola contaminated fluids that dry on a surface remain infectious for 51 days at 39F. Absent a full range of inputs, you plan for the worst case. Underestimation has a deadly consequence.
Well, now that the CDC has told everyone 21 days, they have to let them out of quarantine because they will look really bad once again, with all mixed messages they have been putting out. They will save face instead of quarantining them longer.