Because interest rates would probably have to move quite a bit to bring inflation down and no one at the time knew by how much, rate movements needed to be less discretionary; one way to achieve this was by changing the focus of policy to rates of money growth.
The memory of the 1970s and the subsequent disinflation are very much alive at the Federal Reserve and, I suspect, at every other central bank. What major lessons have we drawn from that era? First, low and stable inflation – effective price stability – is a necessary condition for the economy to realize its full potential for sustained increases in living standards.