Simply a lot of uncertainty what kind of movement it will have around then. Will it get onshore right away? Will it stay onshore? How long could it possibly stall? The NOAA G-IV keeps doing missions so the global models can hopefully give us an idea, but it hasn't made anything abundantly clear.

"Harvey is expected to produce total rain accumulations
of 15 to 25 inches and isolated maximum amounts of 35 inches over
the middle and upper Texas coast through next Wednesday. During the
same time period Harvey is expected to produce total rain
accumulations of 7 to 15 inches in far south Texas and the Texas
Hill Country over through central and southwest Louisiana, with
accumulations of up to 7 inches extending into other parts of Texas
and the lower Mississippi Valley. Rainfall from Harvey may cause
devastating and life-threatening flooding."

35 inches? That would be simply catastrophic. Increasingly, wind and storm surge are also major threats. If this goes as forecast, this will have a devastating combination of these. Usually one or two might be really bad. All three could be devastating here, perhaps historically so.