Where are you getting this information? I see that the Duffer Brothers were still on as producers and writers for season 2.

What I am reading is that they originally intended for the show to be a one-off or an anthology, but once they realized that they had a massive success, one that relied heavily on the likability of the kids, they changed their mind and started working on a sequel. Season 2 sucks because they were never intending on making it, and had a time crunch. With the later seasons though, they've known they were going to make them for years, and they had time to write better interconnected stories, so they've been improving with season 4 and hopefully 5.