Troll thread is trollish. It's like asking why someone would play a Dwarf Commoner versus a Dwarf Noble... they're different experiences, different expectations, a different cultural tableau.
Most City Elves are probably andrastian - since why would humans allow them to practice their old religion in andrastian cities? Andrastianism has never been very accepting of other religions. Even so, some city elves could still remember the old elven gods - after all it's not unheard for dalish elves sometimes to come to live in the alienage (like Merrill).
Also many of them, you know...
believe in Andraste and don't consider ancient elven religion to be "theirs" anymore than the worship of Korth or the Old Gods.
edit: I should add that I understand oppression of religious minorities, it's just that this post seems to imply that if Andrastian elves knew more about the Dalish concept of the ancient elven gods, they'd all want to convert...