In my opinion...

My theory in how this movie developed was that it was never planned to fit in the timeline with the rest of the film. It was mad independently of the others under the "Fast & Furious" title but they found that the character of Han was a popular one so they somehow had to figure out a way to bring him back and make him part of the F&F gang. They then chose to make it a flashback movie so that Han was no longer dead and could join the gang. They then built an alternate story around how Han died. Maybe this has been brought up before or revealed by the creators but that's my theory.