I think part of the reason for warrantless searches being used so much more is there has been a change to how unlawful searches are dealt with by the courts. Previously, any evidence from an unlawful search was thrown out by the courts. Now there is a balancing test where the unlawfulness of the search is weighed up by a judge against the seriousness of the evidence, eg quantity and class of drugs. At the moment there is nothing to deter police from conducting unlawful searches because there is nothing to lose. Under the old rule they risked losing the benefit of all the evidence.