With video games nowadays, it's becoming increasingly difficult to form a consensus on whether a title is worth the money or considered complete, but DoA6 has the issue of being most comparable to the initial release of its immediate predecessor, a game people also complained about lacking content. The Western marketing for DoA6 certainly didn't help and neither did the lack of clarity up to launch, around what would actually be included in the base game. Additionally doing no favors was the grind associated with unlocking costumes and what until recently was the lack of online lobbies.
This is to say, I'm not surprised to see DoA6 perform worse than the last release of DoA5. I would note, however, there really wasn't any reason to buy physical copies of this game in the West, which may have hurt retail orders.
We know based on comments from the game's producer/director that DoA6 didn't have a significant budget, while I've also seen some series diehards pointing to its credit and suggesting a lot of development may have been offloaded to Koei Tecmo's subsidiary in Taiwan. Considering DoA5LR is essentially a last-gen game that visually has gotten long in the tooth in comparison to the series' immediate competitors, DoA6 seems like an intermediary product, made on the cheap, to maintain the fighting franchise's visibility, until a true next-gen entry can be released. The question will be long term whether the negativity surrounding the game was worth whatever profits were made by Koei Tecmo.
Personally, I just want Koei Tecmo to reverse their stance of not allowing Westerners on the PS4 and Steam users to purchase individual costume DLC; unless this changes, they've lost me as a customer for this current iteration of the product.