And there's no point in using Lancasters for EVERYTHING. Plus that not all crews were trained in Lancasters.
Still maybe it would have been more ideal for the Lancaster to be used in Overlord, just before Operation Neptune to bomb those damn bunkers, since the bombers assigned failed.
I'm not sure. I did some checking and I can't find an instance of one being dropped before the invasion. I'm under the impression that it was shortly after that it came into service.