I know you won a DD and all, and that's probably the only reason why I am here to see this, but that being said, I think it'd be a lot more moving if instead of focusing so intently on the imagery they are surrounded in, focused more on the memories they shared.
Even if you've never had an imaginary friend, it's not that hard to 'imagine' as it were, what kind of things you may have done as a kid that made sense to you at the time, but as an adult may have seemed ridiculous. For example, I remember when the movie starship troopers came out, me and all my friends ran around the playground at my elementary pretending to be a squad of soldiers taking out giant alien insects with our state of the art weaponry. I think most of us "played pretend" when we were kids growing up, and if you have too, you could pull on that as a reference.
If you focused more on the memories shared, I'm sure you could even get people who've never had an imaginary friend like yourself, crying by the time you made it to the last line. If you need a movie for reference, you could maybe check out the film "Drop Dead Fred" as the entire movie is about a grown up girl, who meets her old imaginary friend, and how she finally grows up from her childhood, late in her adulthood.
I'm a writer, I should know, and no it's not "harder". There are different styles of writing just as there are different styles of art, and if an artist for example only draws cartoons, they have not explored the technicalities of drawing live people, that could greatly improve their technique.
As far as using imagery, no, it's actually pretty easy, since all you have to do for imagery is think about what physical qualities of the world you find you'd like to describe and then describe it. It's much harder to convey and describe feelings, or personal philosophies without sounding like a fool. So in fact, the truth of the matter is in comparison to describing things we cannot touch, see, or often explain, looking out your window and writing that down is much much easier.
That aside, since you clearly weren't paying any attention, I said that if they didn't focus so intently on the imagery and focused a little more on the memories shared, they could have likely drawn more people in, or even had people crying by the end. As a writer, incorporating more than one things is very important, and they were obviously leaning too heavily on the imagery side, while the imagery itself was sometimes repetitive or redundant. If they gave background to these characters, so we knew what they were like, and what they did, everyone would likely become attached to them and their exploits, so that by the time you got to the end, you're bawling because now you have empathetic feelings rather than a scene with very little to relate to.
As for why SHE won the DD it doesn't really matter to me. Winning a DD is everything and nothing. It's a brief splotlight moment while it can also be the break through people need to get more known. All in all, it's nice, but not that big a deal.