1) Because the moment you make this stuff reserved, you're going to have people who want to take control of their group, usually benefiting themselves more than others. They'll use the ultimate excuse of "Well I formed this group, so if you ever want a chance to get your coins, you'll do as I say" and things become too...businessy. It happened with Dynamis, it happens in Abyssea, it happens pretty much everywhere there are desirable drops that require a lot of members to get. WoE is an exception in that everyone has an equal chance, regardless of anyone else. Just revamp it so the coins aren't as hard to get.
Maybe every participant in the fight should (so long as they have engaged a target) get a chest at the end, and the higher up you are in rank, the more you get in your chest or the better your chances of getting coin pouches.
