Because if it would've released on all major platforms, it would've most likely made triple the income for them.
Those mobile users didn't vanish from one day to the next, so they knew revenue and income was dropping like crazy and with the flop that forspoken was, the situation only got more dire, and yet they chose to release the only game that had hopes of generating a huge amount of income for the company on one console only.
IMHO FF16 isn't a failure as a game, even when some try to frame it as that, it's more a failure to capitalize on the huge hype and demand there was for it after the drought and bitter taste left by FF15.