I think the white background probably meshes better with other items in a treasury. It's also possible that people liked including the message in their treasuries besides the items themselves. But honestly, unless you notice a difference in terms of sales, I wouldn't worry too much about treasuries. The two don't always go hand in hand.
Your work is adorable, and I think the simple but appropriate light backgrounds with text help to give you an identifiable brand. It also shows that you put more thought into how you present your items, I think.