Conversation so far:
What is the Shadow Chancellor of the Exchequer in the British Parliamentary system?
The Shadow Chancellor of the Exchequer in the British Parliamentary system is the member of the Shadow Cabinet who is responsible for shadowing the Chancellor of the Exchequer.
What did George Osborne do as Shadow Chief Secretary?
As Shadow Chief Secretary to the Treasury, George Osborne acted as the primary opposition to the Chief Secretary to the Treasury, who is deputy to the Chancellor of the Exchequer.
Did George Osborne's flat tax idea gain traction?
The idea of a flat tax was not included in the 2010 Conservative Party manifesto.
Was George Osborne's flat tax idea opposed?
The idea of a flat tax was not included in the 2010 Conservative Party manifesto.

Question: Why wasn't the idea included in the manifesto?
Self-contained rewrite: Why wasn't George Osborne's flat tax idea included in the manifesto?