I think this is mainly because it hasn't received a great deal of airtime or print coverage in the US so far, and has thus remained a low-key story. They figure (rightly) that they ought not call attention to the memo by addressing its content.
Incidentally, the memo is highly unlikely to be a Rove plant. It came to light as part of the recent UK elections, appears to be authentic (as far as it goes), and it hurt Blair's party, esp. in Parliament. A President who's approval rating is in the low-mid 40s can not afford a big stink over something like this.
SW
I doubt that any of the memos have been Rove plants. It doen't stop the libs from trying to blame him for their willingness to believe anything that might be negative against a republican.