I was speaking modeling world. Even as a modern US armor builder, I realize that the Tiger tank, Panther and various panzers are more popular than say an Abrams, Bradley or Sherman tank. Any of which out sell any sort of artillery piece, be it towed, self propelled, armored or exposed.
Even the more recent arty pieces, like the Dana, PzHbzt 2000, and some of the various WW2 German howitzers rarely make it to shows. At shows I've been to the armor categories are still over populated by tanks.
In the real world, not including artillery is like trying to play a football game without the forward pass. Tebowing can only get you so far.