Easy. Owners migrating away from your platform to others. 3DS is aging, it's not hard to imagine people have been moving off to mobile for example.
If you can keep your userbase engaged, it might be a sound assumption that content consumption should always increase over time, but that initial condition seemingly isn't an automatic given.
The release schedule is also winding down. If there's very little to buy, obviously download revenue will be decreased.
It's obvious that 3DS is near the end of its life.