The characters & content in the game don't pay for themselves is the quick take on it. Whether the consumer is expected to know the details behind the game's development or care about it is another argument altogether though. Nonetheless, if you're curious and feel like you need to know, here is Harada's explanation on it taken from his Twitlonger post recently.
Harada on the Season Pass ≫ http://www.twitlonger.com/show/n_1spif4h
▌Harada on the Season Pass
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In the past, we had to stop all updates and support within 2months after launch(Because no development Budget).
With Season Pass, we can guarantee one year of Additional Development Cost and development Team (Retention of development team structure) from our Company/Publisher.
Also, we do not sell classic Tekken characters, Paid DLC will be special content. With this system, we can do free updates and support for a long time.
This is what I've been saying in an interview article (I have talked about it in several interviews before, so please read it).
Header image taken from Game Watch.