DmC didn't sold despite having a solid franchise title behind. What is its excuse? it should at least got to 1.8M but thats a pipe dream now.
I already knew gameplay is the main aspect....not sure why we keep having this conversation lol.
Anyways the reason why DmC did bad
1) Dante's design and the overall design of the characters and style is completely different from the past games.
2) Ninja Theory developed it (if you want to search up their history l0l)
3) DMC4's lack of success compared to its previous games - DMC4 had a lot of issues that turned people off from the series and that made them go out to other series such as God of War, Darksiders, etc. The next DMC game was bound to sell far less than this one.
4) The gameplay is far easier than the other games although this point goes head to head with #1.