Why is the Upside Down frozen in 1983 and 1979?

More than any other season, season 4 of Stranger Things was full of plot twists and heartbreaking twists that will have a lasting impact on the fifth and final season.
While some twists were mind-blowing (can anyone say Henry Creel/One/Vecna?), others were subtle but still important. One such twist was the discovery of an Upside Down seemingly frozen in time.
In “Chapter Seven: The Hawkins Lab Massacre”, Nancy, Steve, Eddie and Robin travel through the gate in Lover’s Lake – affectionately called the “Watergate” by Dustin – that Vecna created when he killed Lucas’ teammate Patrick. There they went to Mike and Nancy’s house to collect weapons so they could kill Vecna. Except when they got to Nancy’s house, they couldn’t find any weapons, at least not where Nancy had left them in the real world.
Even more confusing than the lack of weapons was the presence of things Nancy had thrown away years ago. As she rummaged through her things, she found old flashcards from sophomore year, old wallpaper that had long since been replaced, and, worst of all, diary entries that had stopped on November 6, 1983.
“I think the reason my weapons aren’t here is because they don’t exist yet… This diary should be full of entries, it isn’t. The last entry is from November 6, 1983 – the day Will went missing; the day the gate was opened. We are in the past.”
Why is The Upside Down frozen in 1983 and not 1979?

Why do we assume that the Upside Down should be frozen in 1979 and not 1983, if it needs to be frozen at all? Well, this is the year Eleven blasted Henry Creel/One through the crack in the wall in the Rainbow Room. Whether this created the parallel dimension or not remains to be seen, but that was the assumption. However, the fact that it’s stuck in 1983 means that its supposed creation is hardly as important as the day it became inextricably linked to Hawkins.
Eleven didn’t really make it to the Upside Down in 1979
A possible explanation for Upside Down being stuck in 1983 could be that Eleven failed to create a goal in 1979 when she shot Vecna into the dimension. A goal suggests a trip in both directions, but the crack in the wall in the Rainbow Room suggests it was more of a push than an opening. This gap never expanded into a gate, at least not as far as we know, so it was more of a small opening that closed behind Vecna.
The creation of the first goal by Eleven is more important than Vecna’s banishment
It’s possible that when Eleven contacted the Demogorgon in Season 1 and created the first permanent gate, The The action froze the Upside Down. As we saw, the escaped Demogorgon was covered in a slimy cacoon when Eleven happened upon him. This could be a metaphor for the birth of the parallel dimension we know; a dimension that reflects Hawkins.
Stranger Things Production designer Chris Trujillo confirmed this in a featurette, saying that Eleven’s creation of the gate is directly related to the freezing of the Upside Down.
“The moment Upside Down was inadvertently “created” by Eleven, the set and world of Upside Down are frozen in that moment. So in the Upside Down, when we’re in Nancy’s room, we’ll find that Nancy’s room is exactly the same as it was in the first season when we were first introduced to it.”
“The Upside Down” wasn’t “Upside Down” in 1979.
Third, and even more confusing, is the possibility that the Upside Down as we know it is just a manifestation of Vecna’s spirit. It didn’t look like Hawkins when Vecna first arrived; it looked like a barren wasteland. This would suggest that it is not a parallel dimension at all, but rather shaped by Vecna’s own perceptions. Eleven’s opening of the first permanent gate and Will’s disappearance could indicate two potential weaknesses that were helpful in killing Vecna, as both played a role in the day that time stopped upside down. If so, that could have a big impact on Season 5.
It all comes back to Will
The Duffer brothers have consistently said that Will is an important part of this Stranger Things Puzzle. In the season four finale, Will clearly still has a connection to the Upside Down – when he feels the Upside Down’s presence, he gets goosebumps on the back of his neck. This is not a flimsy detail. When season five rolls around, we can probably expect a major plot twist that will blow the entire plot wide open, and I’m betting that Will will be at the center of it. The Duffer brothers have already confirmed that Will will play a big role in Season 5.
Will we ever get an answer to why The Upside Down is stuck in 1983?

The Duffer brothers confirmed that Season 4 would lean heavily on the Vecna storyline, but Season 5 will primarily revolve around the Upside Down itself. “What we haven’t really discussed yet is the Upside Down,” Ross Duffer said Collider. “The big revelations coming in Season 5 are really about the Upside Down itself, which we’re only beginning to hint at.”
The Duffer brothers also said that Season 5 “prepares us to come full circle back to Season 1.” While there may not yet be a clear answer to the question of why the Upside Down was frozen in 1983, you wouldn’t bet too much on assuming it had something to do with it: Eleven opening the first gate, Will’s disappearance, and the subsequent infection.
By the time season four ended, the Upside Down had fully spread to Hawkins, so it makes sense that the alternate dimension will be the focus going forward. We’ll have to wait a bit for our questions to be answered as Season 5 is still in the early stages of production, but as soon as we know the answer to this puzzling question, we’ll let you know.