This one isn't due to extended game development time really. They showed an ES6 teaser (just the logo really) at the same time they announced Starfield in 2018. They had just started working on Starfield at that time and had no plans to start ES6 until it was done. That's about now, ~5 years later. Expect ES6 to be out in 5-6 years, which is in-line with previous BGS games.