They had to cancel the Mixed Relay at the IBU Cup in Obertilliach too due to only being able to secure the course for the Single Mixed Relay - the sprint events yesterday descended into farce as the conditions got progressively worse. Luckily it wasn't quite so bad in the World Cup with one race each day, so the women weren't skiing on trails that were already destroyed from the men's race earlier. However, heavy snowfall was the problem there, similar to in the Nordic Combined in Ramsau, which was likewise seeing athletes stomping through a mountain of fresh snow (plus it also led to a more interesting race than most that we've seen recently, with Riiber put under pressure and eventually defeated by Vinzenz Geiger, and the as-yet-not-at-their-best Germans putting 3 in the top 4).
To be fair, though, with both venues (less so Obertilliach as that's at a reasonably high altitude), there is history for this. This is the hardest part of the season to secure and that's why they tend to go for high altitude venues and Scandinavian ones in the XC World Cup. In 2011 Le Grand Bornand was supposed to join the World Cup calendar in this exact calendar spot, but had to cancel due to lack of snow, leading to Hochfilzen hosting two weeks back to back. They have run in a mid-late December spot successfully in 2013 and 2017, while we've seen worse conditions dealt with at Oberhof and Nové Mesto before too with lack of snow (plus the Pyeongchang World Championships in 2009). As for Planica, I've not seen it like this for an actual event, however I went to the IBU World Cup in Pokljuka in 2015, and we took a trip on a non-competition day to Planica, which was in this calendar slot (16-20 December), and there was no snow in sight - Pokljuka had enough snow, just about, because Pokljuka is up at around 13-1400m, but Planica was entirely barren of snow and pretty much all snow that they had in Gorenje at that point in time was being used to either keep the IBU World Cup in Pokljuka running or to keep at least some runs at Kranjska Góra open.