>Then it was never "popular"!<
Thank heavens! This is a breed that would, imho, be tragic if it became popular as a pet. These dogs have a lot of drive, and they would end up in dog pounds by the hundreds, after shredding the family couch, or the woodwork. Belgians need to be kept busy, and they must be trained, especially when they are puppies/adolescents.
My dog has the habit of standing behind the couch, then launching himself over the back, landing beside whatever human happens to be sitting there, minding his/her own business. When we go to the Vet, he's been known to jump up onto the counter, to say hello to the receptionist (who by then has doubled up in laughter).
But if it was popular, mutations would emanate. Many would probably not have as much drive. Lord knows this and many more things happened to German Shepherds after going popular (which was early).