I also wander about the lack of advertising. I came up with the following explanations:
1- Nintendo has no money to spare since profitability is key at the moment
2- Nintendo was testing the waters, and is holding back their marketing machine for a big push (Wii Fit U, ....)
3- They thought the minimum push was really enough for it to pick up
The next months will reveal the reason... or even others...
In the wilderness we go alone with our new knowledge and strength.