Whats the cause of this? quality or series fatigue? or lack of marketing, co-marketing?
Could just be the first entry being incredibly inflated. It might not be fatigue as it is traditionally seen, or even a sign of the (lack of) quality. Definitely not lack of marketing. Just because you can get people to buy a lot of one game, does not automatically mean that you can retain the entire audience for future entries. Especially for a semi-annualized series.
Series fatigue only applies if a franchise consistently sells a certain amount, and then starts seeing sharper declines later on. An example of this would be the Layton series or the Inazuma series. Gran Turismo also exhibits this syndrome, but that could also be genre fatigue, with Japanese audiences being less interested in full price racing games.
With One Piece Musou it seems likely given the given the figures that there are probably about 300-400k worth of buyers who are really into it. A lot of the sales in the first entry were probably people who don't regularly buy Musou games or One Piece games, but liked the idea of One Piece being made into a big Musou title. They tried it out, and that was enough for them.