@Yoavush7 I can only speculate as to why. If I had to guess, I would assume it's because an exchange system would make it sort of pointless for people to buy more than one. If you can exchange the heirloom you have, you can then just trade it in for which ever legend you decide you want to play. That's just a guess as to why..
In terms of purchasing the heirlooms in game, if you have shards, you have to verify that you want that heirloom multiple times. So it's not like you're accidentally clicking purchase.
I think the one exception would be for players who received their heirloom before shards were implemented.
