As part of the 2017 UA Artificer's Gunsmith subtree, you gain the Thunder Monger ability. Why would a player ever choose to not use this ability vs a normal attack with the Thunder Cannon? It always increases the damage done by the Thunder Cannon, has no "X per day" caveats, and (seemingly) no negative repercussions. So with an unlimited number of uses and no down-sides, why not just always use it?
I ask this not as an opinion, more a clarification. I want to be sure there isn't a rule I am missing or effect I am misunderstanding. Also, I am aware this is a UA class and therefore still under playtesting, but I want to try this class out.
Because it is just one tool in your toolbelt
The Artificer is also a 1/3 caster, like the Arcane Trickster, and many of their spells require an action to use. Therefore, using Thunder Monger every round rules out spellcasting for that round.
At higher levels, the Artificer gains other abilities that consume their action: Blast Wave, Piercing Round, and Exploding Round. This competes with Thunder Monger as they all consume the Artificer's action.
Finally, there are a lot of other default actions aside from Thunder Monger: Ready, Dash, Disengage, Help, etc. When it's more important to run away, Dash or Disengage will help the Artificer more than Thunder Monger will.
Thunder Monger vs Attack action
You may be specifically asking "why use the Attack action over Thunder Monger?" Well, consider that this is like asking "why not use Sneak Attack every round?" The answer is: you should always use Thunder Monger at the level you gain it, if your choice is either the Attack action or Thunder Monger.
The Artificer is not a full spellcasting class, so it needs some way to contribute damage. Just like the Rogue Arcane Trickster, this subclass of the Artificer uses extra d6's on a hit to catch up with the Extra Attack from the other classes. Notice how similar the progression is between this feature and that of the Rogue's Sneak Attack. This shows that the extra scaling damage is balanced, as another class already uses a similar mechanic.
It is also worth noting, if you multiclass and gain the Extra Attack feature elsewhere, Thunder Monger and Extra Attack will clash. At higher levels, Thunder Monger might deal more damage, but Extra Attack will be the winner in terms of damage at lower levels, assuming a high enough Dexterity modifier.