Reasons: 1st half of 2019 being weak in new games, like 2018 was. In the 2nd half of the year Animal Crossing, Pokemon 2019 (if it launch in 2019, something that has yet to be confirmed by the way), Fire Emblem, Luigi's Mansion, and every other game you always name....not surpassing Smash Ultimate, Pokemon 2018, Super Mario Party, rest of games from 2018 saleswise compared to 2017 ones. So in the end, as whole for the year, a good lineup, but not neccesarily better.
Here are the sales for the relevant 3DS titles:
Pokemon X/Y - 16.37M
Pokemon Sun/Moon - 16.14M
Pokemon Omega Ruby/Alpha Sapphire - 14.17M
Animal Crossing: New Leaf - 12.10M
Super Smash Bros for 3DS - 9.45M
Luigi's Mansion: Dark Moon - 5.75M
Fire Emblem Fates - 2.85M
Mario Party: Island Tour - 2.68M
On Nintendo's previous handheld, Animal Crossing was bigger than Smash and both Luigi's Mansion and Fire Emblem were individually bigger than Mario Party, so it's strange how confident you are that 2018's lineup has more selling potential than 2019's. Unless you think that Pokemon Let's Go will sell more than Pokemon Gen 8, but I don't think even the most desperate detractors would argue something like that.
I'm very curious: How much do you think 2019's games will have shipped by the end of the year? In particular, Animal Crossing, Pokemon Gen 8, Luigi's Mansion 3, and Fire Emblem: Three Houses.