I did not think i would need to spell it out any more clearly but the marketability/sales ability of the game equals potential revenue/potential profits
Thwe market trades on a forward basis of what it thinks can/may happen in the future and when it may happen- a key product the market hopes will bring in revenue growth and or profits in the future that is disapointing to the maret causes a drop as much as a delay perhaps more-
The delay and the sales/marketablity (real or percieved) and the earnings miss combined hurt the confidence of investors -
PS: When I say investors "care" about the game it is in terms of thier confidence in it not weather or not they may like to play it themselves- They "care" very much just not in the same manner a gamer or Nintendo fan may "care"
That makes more sense but the nature of the app is more of a factor then what you say because it's free for a start, investors were hoping for a big IP to be sold to consumers for more immediate returns. Miitomo is more to bring people on board to the new account system and acts as a hub for apps and games, it connecting to consumers won't be an issue it just so far hasn't shown how it'll bring in profits and that's what has investors backing off for now.
The sales ability is not the issue it's that it's not what investors were initially hoping for to begin with, it's nature and delay means expected profit won't come as soon as expecting because the app is a long term plan to build a dedicated number of users as opposed to selling minigames and microtrasactions for immediate returns.