I wonder why it was then and not the moment El opened the portal to banish Henry through it
Because the Upside Down didn’t exist yet then. It was still just the chaotic Dimension X. It was only after Henry transformed into Vecna that he gained some form of control over the dimension.
El making contact with the demogorgon is the first such dimensional contact since she pushed Henry through. This contact is what enables him to find a way to send the demogorgon through.
It’s possible Henry/Vecna didn’t intend for the Upside Down to mirror a specific point in time. It may have just happened that way.
(As an aside, based on the revelations in the First Shadow play, which have been hinted at in S5, I’m of the mind that Henry/Vecna is
not the BBEG. Either the mind flayer is still controlling Vecna, like in the play, or there’s some other entity that is the master of the hive mind, that is controlling everything in the Upside Down: Vecna, the mind flayer, the demogorgons, the demobats.
Perhaps it is the three-headed dragon thing that appears in some of the promotional matter. I’ve seen people suggesting the kids will dub it Tiamat.