First of all, they did not spend 6 years on it. 5 years since BLTPS came out. thats when you can assume production first started. Edit, actually it started 4 years ago.
HOWEVER, Battleborn came out in 2016. that means a large portion of their workers were working on a different project. after it came out, a large amount of workers went from Battleborn to BL3. But a significant portion stayed to balance the game, and create new content (If they had abandoned the game, they would have scammed a lot of people).
We can assume the last people moved over in 2017 or so. So production was not at full speed until 2017~.
Second point, they changed engines.
The new unreal engine was designed for full power, omega graphics, and it did not care for performance. So, unlike in BL2, they could not just grab the models from BL1 and improve the graphics, they had to create entirely new models for characters and guns. Plus, since it is a brand new engine, they can’t just look up “what is causing this problem”. It’s like us and finding the new guns. if we want to find out where things are, or what they do, you have to find it yourself. And, they switched engines after a year of development.
And the last point, they changed the story late in production
They have stated that Handsome Jack was going to come back in this game, but they changed their mind, because they didn’t want Borderlands to just be “Jack”. Yeah, we love him, but if they went with jack for the 4th time (BL2 and BLTPS, plus Tales), It would just be reusing him. So they changed the story pretty late.