Really cool, the only thing is that it's too horizontal and thus the T shirt looks a bit empty. But it's only my opinion and I already find this great!!!
I don't see why this was rejected, but I'll give you some suggestions anyways.
The worms are inconsistently lit in relation to the apple.
Take advantage of the blue print graphics could actually say "gummy worms" or something clever. I can't read them so maybe you did. I would actually read them if I was to buy this shirt.
Experiment with placement. It is placed in a very typical area. You may find a way to really bring the design out. I can see threadless doesn't just want shirts with cool designs, they want designs that use the shirt as a canvas.
I want to submit this for the Pre-Loved challenge
Thanks