It may have been because the adjustment pressure on the economy was, and is, still quite strong.
However, one of the effects expected from the introduction of quantitative easing was the so-called “portfolio rebalancing effect.” The Bank thought that, even when the marginal value of liquidity services became zero, people would start to rebalance their portfolios by investing in assets with higher marginal values whether these were real or financial assets, if the Bank increased further its provision of liquidity.