As already explained by others, 99% of the 747's are cargo aircraft indeed, so 25R suits their needs better than having to land on 25L and taxiing all the way to the Brucargo ramp (I think that they'd even prefer going into a holding pattern for 1 or 2 turns then landing on 25L when there's much traffic on 25R). Nowadays,
BRU spotters will only see 747's land on 25L when there's a lot of departing traffic, 25R is closed (the whole length or only a part) for maintenance works (or some inop ILS etc) or when the 747 is a pax aircraft (Egypt Air, Royal Air Maroc, or even an ad-hoc special charter such as the
BA 747-400 a couple of months ago).