Why do some countries prefer UHT milk and cream?
My guess is that the Belgians (like the French) rarely eat milk cold and uncooked in the way that the British do. The Belgians will have their milk in hot chocolate, or cakes etc, whereas the Brits will have it cold in cereal (again, in France at least, cereal is eaten with warm milk).

I have found that the UHT milk in French supermarkets tastes a lot better than the UHT milk in British supermarkets.