It's not a good reason, but it's a reason
Many of my friends were outraged when Greedo shot first. I just read this explanation. I do not know if it's true (it's from a Wiki), but it pleases me to think that there was a reason.
"Apparently the whole ordeal was due to changes demanded by the MPAA. The PG-13 rating was created after the original film, and when Lucas submitted the special edition cut to the MPAA he had to change the scene to avoid a PG-13 by not having Han shoot first."
Source: http://tvtropes.org/pmwiki/pmwiki.php/Main/TheDogShotFirst
"Apparently the whole ordeal was due to changes demanded by the MPAA. The PG-13 rating was created after the original film, and when Lucas submitted the special edition cut to the MPAA he had to change the scene to avoid a PG-13 by not having Han shoot first."
Source: http://tvtropes.org/pmwiki/pmwiki.php/Main/TheDogShotFirst