My speculation is that they wanted to hit a 2020 release because the name of the Cyberpunk 2020 edition of the original RPG, and the fact that Cyberpunk 2020 celebrated its 30th anniversary in 2020.
They rushed to meet deadlines and got burned by the scope of the project.
Problem with C2077 is verticality of the game, Witcher was laid-out horizontaly on large plains. C2077 must have been optimization nightmare especially when you suppose to down-port it to last gen consoles.
They even sold cyberpunk themed limited edition Xbox One X, a cyberpunk themed PS4 controller.. so I'm guessing the pressure to release for these consoles before they're completely obsoleted by the new gen must have been immense.