BIH are selling the PZE site to raise capital to buy new machines to support their North Sea oil operations (without which they claim they can't support the wider business). I presume St.Erth will be leased hence creates the necessary capital.
BIH tried to relocate to Lands End, but the owners rejected their advances. Frankly Lands End would have been a disaster because of the greater levels of fog.
I think Newquay was considered, but imagine a prime consideration was to minimise the extra travel time. BIH claims the journey will be about 2-3 minutes longer. They've not said anything about fares going up yet, but watch this space.