the wihi page on Lambic says: "Unlike conventional ales and lagers, which are fermented by carefully cultivated strains of brewer's yeasts, lambic beer is instead produced by spontaneous fermentation: it is exposed to the wild yeasts and bacteria that are said to be native to the Senne valley, in which Brussels lies. It is this unusual process which gives the beer its distinctive flavour: dry, vinous, and cidery, with a slightly sour aftertaste."
So if that's true, the explanation that you can only make it there is pretty spot on.