Although not depicted in the film, according to the Star Wars movie novelization ghostwritten by Alan Dean Foster and the comic adaptation, Chewbacca did receive a medal.
From the comic:
From the novel:
She placed something heavy and golden around Solo's neck, then
Chewbacca's--having to strain to do so--and finally around Luke's.
Since from the Expanded Universe's perspective, Chewbacca did receive a medal, you are unlikely to see an explanation outside of the films for why Chewbacca does not receive a medal on-screen.
If you want documented evidence of Carrie Fisher placing a medal on Chewbacca on video, Chewbacca got a MTV Lifetime Achievement Award in 1997. This award ceremony was detailed in Star Wars Insider #35 "Chewbacca's Lifetime Achievement Award" by Athena Portillo (page 28), along with a brief mention to your question.
Was the 7'2" Chewie too tall for the petite princess to reach during
the ceremony? or...Was the medal too small and not accommodating
enough for such a furry Wookiee? The answer? Simply none of the above.
It just wasn't written into the Star Wars script.
The answer from Star Wars Insider is probably the best answer you will get from Lucasfilm.