Because it involves something that the Bank of England and orthodox central banks almost never do - which is to lend money with the express purpose of getting it out into the real economy, rather than just providing the marginal credit that commercial banks need to operate efficiently.
This idea has been championed by conservative policy entrepreneurs such as Ramesh Ponnuru of National Review and Robert Stein, a veteran of George W. Bush's Treasury Department, yet it has met with fierce resistance from The Wall Street Journal editorial board, which sees it as an unjustifiable tax giveaway that is an unhelpful distraction from the need to cut marginal tax rates.