Walibi Holland doesn't even crack 900,000, to be fair (and in double fairness, Holland's location is pretty crap and its focus towards teens and adults limit how high numbers can go a bit). I think Walibi still being seen as a mostly regional brand plus Belgium's lower tourism numbers compared to neighbouring countries play a part in it.
I think that Walibi's franchise tends to target teens and adults in general, i would easily add CDA's jewel Parc Asterix to the list as well. WB has probably the biggest potential for the future developments (and always had) but it was simply missing the big catchy investments to truly give credibility to the park, now it's done.)
At the same time WB IS located in an area already super dense themeparks related. (Plop and Efteling already have the child target, Bellewaerde/Toverland/Bobbe/WH have their familial or young local crowd etc.)
IMO Belgium isn't on the map yet when it comes to amusementparks, but it is going to change for sure ! . Belgium probably has a lower tourism rate than other close countries, but Festivals and stuffs brings millions of people in Belgium during the summer season for decades, we're talking about young, exited, people who wants to thrill, which is an audience more interesting than the classic tourists you can find elsewhere on the same period. Their best strategy is to invest into thrill rides as much as possible and get a prestige from it.