Because they are services hosted outside the UK and which are not heavily regulated. Banks not only provide everything the gvt asks but actually have a duty to act as a law enforcement agency, i.e. watching their customers on the gvt behalf. Facebook or Gmail aren't required to do that.
You seem to imply that currently (retail? investment?) banks in UK are obligated to provide the government with information regarding customer's accounts. Can you provide a reference?
