Each agency has their own preferences - Veer in particular has some very odd notions (IMO) of what they view as having commercial appeal (having turned down files that have sold well elsewhere). I can see from your SS portfolio that you have a lot of these characters with no features. It clearly has a style, but it's quite simple. iStock seems not to like vectors that are very simple - although they've certainly accepted others of yours with that style.
As far as a specific critique, I don't like the tree much. I think the key shapes - baubles and lights - get a bit lost in this outline treatment. Perhaps a tree where you could clearly see more/all of the ornaments hanging down would look more Christmas-y?