There are a ton of cards that care about enchantments in both green and white, and cards like Yavimaya Enchantress and Ancestral Mask help to promote building a huge creature that would love to have lifelink and trample
Just like persist on Putrid Goblin, undying is easier to break than a porcelain vase in a Matrix movie
Lorwyn Eclipsed's Kithkin This Kithkin would crack jokes with you over a pint, and I just love the stuff it's accumulated
Legendary Artifact The Ozolith Whenever a creature you control leaves the battlefield, if it had counters on it, put those counters on The Ozolith
There are many niche merfolk support cards that will basically only see the light of day for a product like this, so Wizards of the Coast had no reason to leave any of them in the vault unless they break the blue/green color identity (sorry Sygg)
Heres part of what I wrote about him: Xenagos will be the flagship of a tier 2 deck that occasionally flashes tier 1 success