Again, some of the 7 GOP members had indicated their willingness to use the "constitutional option" in the event the Dems filibustered without good cause. A qualified SCOTUS nominee is likely that kind of good cause.
And "good cause" was spelled out by the Dem members, as I noted - and would allow filibustering over preceived extremism. To the Dems, any professed originalist is an extremist. Rogers had to couch his words very carefully to get around that. Miers doesn't have a judicial history for the Dems to attack her with.