system
Given the following post, respond as if you're commenting on Reddit:
human
[Stranger Things] The Mind Flayer is the most powerful being in the show. So why is it difficult for him to pass between dimensions?

In season 1, the Demogorgon could easily pass between dimensions without having to use the Gate in Hawkins Lab (the Buyer house, the forest, Hawkins Middle School, etc.). 

Eleven was able to open the Gate in season 1 and eventually close it (when it’s much bigger) in season 2. It was by accident in season 1, but still impressive. 

And later in season 3 the Russians made a machine that opened the Gate. Or at least cracked it open enough for the Mind Flayer to reawaken part of himself to enact his plan. 

So it’s *relatively* easy for a portals to be made between the Upside Down and Earth (at least in regards to the Demogorgon). So why can’t the Mind Flayer make a portal like the Demogorgon does?
gpt
Because "power" isn't usually monolithic outside of Dragonball Z. The Mind Flayer might be powerful, but for whatever reason it has a hard time moving fully between dimensions - its own power might be working against it here. maybe it won't fit through the smaller gates we see, or they won't last long enough to let it pass through without collapsing.

Example: In Warhammer, larger daemons are more powerful, but are harder to summon and can't stick around as long without the intervention of powerful magic or abundant fuel(skulls for Khorne, warp stuff for Tzeentch, etc), and the big four can't come through at all - reality just can't support their existence.