Because Calman is radical - in terms of tax powers, a big change from what Scots were made to vote for in a specific question on tax in the referendum of 1997. So if you want to change these powers, have a specific vote.
In this regard the SNP is in danger of clouding the issue over its demands that aspects of the Calman Report over which there is "consensus" - those that relate to firearms legislation and speed limits - be implemented now. I see where they are coming from, but it's a confusing message.
"Give us all a vote Jim" is what we should be demanding loud and clear. This is about the sovereignty of the Scottish people, not a cosy "consensus" cobbled together by a self appointed committee and a bunch of politicians.
And of course they fear a referendum - on near anything.