Because if active use is made of it, price stability might take a back seat. The massive increase in government debt in the wake of the crisis could make this problem more acute. Political pressures could arise and grow to keep interest rates lower than the rationale of price stability would call for.
That would be nothing other than ﬁscal dominance as described by Michael Woodford: “In fact, ‘ﬁscally dominant’ regimes often do not involve any direct assignment of a seignorage target to the central bank, as in the textbook analysis.