A ranked balloting system would be seen as an attempt by many to permanently game the system in the Liberals favor. 

Prop rep would magnify the role and visibility of the NDP. And therefore criticism of the Liberals.

Butts may think they are already riding high and sees little upside in opening up this big can of worms. Besides, many Liberal supporters don't get much past the selfies and Vogue spreads anyway.

Politically a shrewd move by the Liberal brain-trust.  And blaming their flip flop retroactively on Harper will hold more cachet with some than we might think.