Tamriel can host a lot of festivities, as seen in the UESP calendar and the Arena calendar. This thread asked for tamrielic festivities too: forums.elderscrollsonline.com/en/discussion/141155/tamriel-holiday-festival-patch-hope-its-added-sooner-rather-than-later.
So, in short, if we hadn't had them in a whole year it's because that is not a priority for ZOS.
I'd like this was on their long list of "to do" things, so perhaps this year we could have a surprise or two concerning this.
I hope so! Festivals bring so much cheering in an MMO. And has been demonstrated that lore festivities do exist. No need to do anything off-lore, we have enough lore-friendly background to do celebrations and/or special events, always lore-based.