They noted that it had stiff competion in the slot (I never checked) and that it lost a significant portion of its lead-in audience. But the critics still liked it...
That's why I think it may get moved to a different slot before they cancel it outright. Remember, X-Files got moved, too.
They're doing an amazing job of providing new plot ideas and twists from week to week. Such a basic concept allows for a surprisingly rich and complex plot.