Probably there is no single reason for these changes. Bordeaux wines are almost always blends from two or more types of vine. Many New World wines are single vine strains such as Cabernet Sauvignon, Merlot, Pinot noir, Syrah (Shiraz), and the white wine grapes Chardonnay and Sauvignon blanc. Bottles labelled by type of grape (and so by taste) has caught on with the public. Also, the cheaper French wines are much more expensive when exported. All wines and spirits into countries like the UK are heavily taxed. A cheap French (or Spanish or Italian) wine which might sell in at home fore under £1, will sell for £4 or more in the UK. Customers have evidently decided that wine from elsewhere is a better deal.