Given the fact that artillery isn't even a thing yet in this endeavor it's no wonder. The same applies for Cavalry, there are not very many of those units either, for the same reason.
But then having said that, and read so many comments and discussions about what a Gun crew should be composed of I sense that the average, and maybe the not so average, or less than average gamer thinks that artillery is dull and boring. General misconceptions that artillery can be manned by two players minimum, or even one, and the like indicate people do not have an appreciation for the career artillerist.
In most games of this nature, player stats are usually counted by personal kills. Players don't want to play a part of a game where they're not getting "points" to some purpose. But the developers have stated that statistics like that are not going to be kept. So individuals won't have a reason for bragging rights in the forums as being the hottest shot in the community. So with the knowledge that players aren't racking up points, I would suspect that for the purposes of authenticity, we might find more players joining artillery units in the future.
Right now, Infantry is all that can be played. So everyone plays infantry.