The problem at hand is the fact is that there is the licensing involved for Sony and Microsoft. With console purchases (and free items from earned Loot Boxes), your items are part of your license to Overwatch on that platform which Sony and Microsoft get a cut of the revenue with. Sony and Microsoft, doesn’t want you to leave their platforms obviously.
This could change in the future, but really its going to come down to the terms Blizzard make with their console partners.