But in episode fourteen, we were finally shown the origin story of one of them: Rarity. Plus, even though its not much, we have still been given subtle hints that that seem like one off jokes, but are likely more: Applejack's delusion that she runs a mango farm, Spike being from the Bad Lands, etc. And unlike most Abridge Series, this one has an story arch, which itself still has a lot of mystery.
Now compare that to how Spike keeps asking questions about the contradictions in pony society and arguing with Twilight over how good Celestia is. Spike never dwells on any one point longer than a single scene, so they don't really affect Twilight that much. Basically, Twilight's still in the "If I ignore it, it will go away" reaction phase—constantly using "It's magic, that's all we need to know," to avoid thinking too hard about these issues. (Although episode 15, where Twi discovers Spike's hate mail and gets so angry she refuses to talk to him, may be the start of the "losing her temper" reaction phase.) So, eventually Twi is going to accept that Spike is right—that Equestria is a weird, contradictory place—and make peace with the absurdity.
So Celestia's familiarity with Friendship Is Magic also gives her vague foreknowledge of her own universe. And that foreknowledge tells her that the mane six are going to solve every crisis for her. In that other universe, she's seen Twilight and her friends defeat Nightmare Moon, drive away the dragon from Bald Mountain, drive the parasprites out of Ponyville, defeat Discord when he escaped, etc. So why fix what isn't broken? When those same problems pop up in her own universe, Celestia throws the mane six at them and waits for everything to work out. And all that waiting leaves her bored, so she passes the time by screwing with Luna—who hasn't seen Friendship Is Magic, and therefore doesn't know the future like Celly does.
But then Sombra and Chrysalis show up and throw a wrench in her plans. Celestia knows from Friendship Is Magic that neither of them are "supposed" to attack until later. Their appearance now signals that her universe is going off the rails, and it invalidates everything she knows about how they're "supposed" to get defeated. There's a distinct chance that they'll actually win. Hence why Celestia takes Sombra and Chrysalis so seriously, in contrast to the other threats against Equestria.
(The one event that doesn't quite fit into this is episode 4. The cow curse on Ponyville isn't foreshadowed by Friendship Is Magic, yet Celestia still reacts to it with her usual nonchalance. My best explanation is that, since Sombra hasn't made his reappearance yet, Celly doesn't realize it's possible for her universe to deviate so severely from Friendship Is Magic. So, upon hearing of the cow curse, she analogizes it to the events of "Applebuck Season"—and since the mane six worked everything out by themselves in "Applebuck Season", clearly they'll fix the cow curse themselves, too. In this case, Celly is Right for the Wrong Reasons.)
- In the early episodes, that was actually Ditzy, Derpy's identical twin sister. And Ditzy has no beef with The Bird.
- Derpy has multiple personalities. The early episodes were her Ditzy persona, who again has no animosity towards The Bird.
- Derpy has really bad eyesight, and simply didn't recognize The Bird for nineteen episodes.
- Or giving her a speaking role for a change, might've changed her personality do to the influence of her voice actor.