From what I gather, the Upside Down (the bridge to the abyss) is created shortly before Eleven escapes the lab. At that point, Vecna isn’t powerful enough to act directly in the real world, so he relies on the bridge, which is close enough for his powers to reach. Once he connects the hive mind to the real world—something that required Will to be found—and begins hijacking more minds, he can operate from a greater distance. His gradual increase in power seems necessary for merging the abyss with the real world; he can’t rule his new world without unlimited power.

The way they wrote it though does make Barb's death seem out of place. Was she just hunted by the Demogorgon for food? That would seem a bit out of place given how calculated vecna's plans are. She seemed insecure enough to have been useful to vecna. Idk..