It's due to the timetable change. Here is what The Man in Seat 61 says about it:
Tip 1, the December timetable change
All European railways change their timetables at 24:00 on the 2nd Saturday in December.
Operators never maintain their usual booking horizons for dates after the timetable change.
This is the Big Annual Timetable Change. There's a minor change on the 2nd Saturday in June, but the December one is the Big One.
So you can easily look up trains on (say) 13 December, but if you try to look up trains on 14 December you'll find no trains show up until much closer to departure date. Or you'll find some trains are loaded but others aren't. Or trains are shown but won't book properly. Whatever. It's all to do with the Big Timetable Change. Got it? Just be patient!
French, German & Austrian railways (SNCF, DB, ÖBB) usually open bookings for dates after the mid-December timetable change in early to mid-October, so the normal 90 days shrinks to 60. And even then, the French only load data until the beginning of January, there's another delay before dates later in January are loaded.
Spanish, Portuguese, Hungarian, Polish, Slovakian, Czech & Bulgarian railways usually don't open bookings for dates after the mid-December timetable change until late November or even early December, so the 60 or 90 days shrinks to as little as 10.
For places such as Turkey or Lithuania, you're lucky if the timetable data is loaded the day before the new timetable starts.
But don't worry, no-one else can book either. The train won't sell out, you'll still see cheap tickets when booking opens. Just wait!!!