That's an interesting question! Zariel was clearly priced to move, but I think the main problem is that her niche just isn't there. She's obviously a tool for either midrange or aggro decks, so let's look at each in turn.
Re: midrange, she's stong but not busted without support, which isn't enough for a midrange 4-drop in Standard these days. Compare her to premier 4-drop standard threats like
Esika's Chariot or
Sheoldred, the Apocalypse--both can take over the board even from a deficit, which Zariel can only kind of do.
In an aggro deck, Zariel might be okay as a curve topper much as
Experimental Frenzy was a few years ago, but aggro decks aren't interested in things over 3 these days. There are enough mana sinks attached to cheaper cards that it makes more sense to run something like
Squee, Dubiois Monarch or
Bloodthirsty Adversary instead, either of which is live earlier in the game and in games that stall out at 2 or 3 lands.
It's probably clear that I like Zariel and that I think more people should consider her! I like that she's a pseudo-anthem in a color that doesn't get many of these effects and that her emblem, while strong, is a little unusual and requires lateral thinking to take full advantage of. (Hint: it works best with dragons.) Her token-making ability doesn't push her towards ulting, which reduces a big frustration factor present in many Elspeths, and the death trigger on the tokens incentivize trading, which again helps clear up boars states. There are a lot of chances to feel clever with this card on both sides of the board, and typically there's an interesting tension in terms of which ability to activate.
