Maybe they didn't have enough money to add an online mode free of charge, so they're doing it this way. Paid online mode > no online mode.
I don't think it's really just down to an online mode. I think there are some things to consider here:
- Fantasy Life is developed by Brownie Brown, so presumably any additional content or extra programming work would have to be contracted out to them as well, instead of a development cost which Level5 can eat internally.
- Simply contracting out an online patch might not be worth the time and money, especially if it adds very little in terms of value to people who already have the game. So the decision was probably made to extend support to the game via the form of an expansion.
- While developing an expansion to the game, they can also add additional features and functionality to the game - like an online mode. As such, it could be seen as one entire package budgeted as such.