Past customers cost P&O money. Peninsular discount, free cocktail party, Peninsular lunches etc etc. Plus as long term cruisers they are less likely to pay for shore excursions (been there, done it, got the t-shirt), rarely frequent the shops or the spa, or bother with photographs. They know the tips and tricks to get extra OBC.
New cruisers do not get discounts, parties and dinners. Much more likely to buy shore excursions, souvenirs from the shops and photographs, indulge in special venue dining etc.
So it is in P&O interests to encourage new cruisers rather than try to keep experienced cruisers.