SFV is underperforming but it's understandable, Ultimate mega ultra super EX SFIV released some months ago and have everything you need in a fighting game: content, stage, characters, almost perfect gameplay etc...
SFV has bad WOM from the start, it doesn't have enough content to hook you in and it's too difficult to get fun online when some people are just killers, I bought the game and all I can do basically was playing online but the servers were broken for days and when I had some fights, I got absolutely destroyed, I don't care much about this licence to continue playing it so I sold it and tell to everyone I know why they shouldn't buy it.
Naruto is more casual friendly and is really fun, it's a gorgeous game too, it deserves its sales.