It's really simple actually. I think that Feige wanted to honor his working relationship with Edgar, and let's be honest this project has been on the backburner for most of that 8 year period. Edgar didn't do much work other than help sit in on meetings where they were developing the special effects and such.
When the project gained traction and they were hiring actors, and things were moving forward, and they had to get a script together, that's when things blew up.
I think Feige was perhaps naïve in thinking that Edgar would be open to change based on how the MCU shaped itself, and Wright was naïve to think that he could make the exact same film he envisioned 8 years ago, before Iron Man came out, before Cap and Thor came out, and before Avengers came out.
Both parties share some blame, but I blame Edgar more for being completely inflexible.