A CAT2 runway system would require a hell of a lot of rock, infill and money to start with- or the Castletown road will have to go as the runway would need extending at least 200m the other side of the road to accomodate the minimum runway length, and a full CAT3 lighting fit in the undershoot for 26.
They could always dig the hill out and make 21 the main runway but that would be impossible to justify with the money spent already.
It's only 3 or 4 days a year after all. That's what the boat is for isn't it?