They were rather niche. Would also have presented some significant modelling problems. Small 4-4-0s can be problematic to motorise and balance - even in 7mm you may not get away with a conventional motor gearbox combo. The GN J-class, for instance, requires a rather expensive 3-stage set of gears from ABC  as pictured. Belpaire boilers make life a little easier but a round-topped firebox rather limits your wiggle room.