What is the secret of Laithwaite's success?

Next year is the fiftieth anniversary of Laithwaite's Wine, the British wine merchant originally known as Bordeaux Direct. According to this FT article, they now sell from an inventory of 1,500 wines, of which 95% are made exclusively for them, and turned over £340 million in 2016. That makes them bigger than Berry Brothers and Rudd (founded 1698) and Justerini & Brooks (founded 1760), so how have they become so successful in the lifetime of the founder, Tony Laithwaite?