Because if the members of the House Judiciary Committee expect US Attorney General Michael Mukasey to comply with a subpoena regarding documents connected to political scandals^*, they have the political acumen of Little Orphan Annie^©.

You have to vote on the subpoena, but once the deadline passes, have him arrested.

^*DoJ foot dragging on the New Hampshire phone jamming, the US Attorney firings, political interference in the Civil Rights Division, etc.