I think Lucas was just really strapped on time and money. I am not so sure a reshoot after the fact would have been possible. I think that's why he just "fixed it in post" by tweaking the dialogue to make it clear that Greedo planned on shooting Han.

Obvioulsy some here didn't think it was all that obvious based on Greedo's body language, which just goes to show the subtitles weren't always enough to get that point across.

I think that's why Lucas felt it had to be clarified.

I can see why people don't all like it, but at least the 2004 SE isn't quite as awkward looking, and they shoot at almost the same time to the point that both shots are on the screen at the same time, with Greedo's just a touch quicker.

To me that timing change alters it from a reaction shot on Han's part, to a shot he was making at about the same time. I always looked at that timing change to be a "okay let's meet in the middle" on Lucas' part.