@Incursor
Of course Equestria didn’t know of the Storm King dominating Mount Aris/Hippogriffia for well over a decade or two. Even Celestia and Luna didn’t know that their long lost old friend have been overthrown by the Storm King and exiled into the ocean. That isn’t until the Mane Six met Queen Novo and Skystar after escaping from the wrath of the Storm King who has been dominating Equestria.

You’re saying they knew because they’re too far away to have known? Or did you mean didn’t know? Or am I misunderstanding your point?

Equestria didn’t know of Storm King concurring the hippogriffs for well over a decade (so long stairs were exciting and new to the teenage Silverstream). By that logic the three days the movie was set over wouldn’t be time enough for the other races or Crystal Empire to have noticed or acted.

Discord was in the credits party, but I don’t think he was there sooner otherwise he would have done something to help the good guys (unless there’s some detail I missed).

@Incursor
Well, I think the other four kingdoms beyond Equestria which are the Dragon Lands, Griffonstone, Yakuakistan, and the Changeling Kingdom, did know that the Storm King have overthrew all four princesses of Equestria because they’re too far away from Canterlot. Also, Discord is still in Canterlot during both the Frinedship Festival and the Storm King’s reign of Equestria. And the Crystal Empire is part of Equestria because it is in the Northern borders of Equestria. The Storm King is now doing something like what King Sombra did one thousand years ago.

@Background Pony #39B5  
@Background Pony #4AFD
 
Because work on it started in 2014 after Season 4 ended. Episodes Have their scrips locked over a year before they air. The large amount of extra-high quality animation means they had to lock the the movies story sooner. Ergo the script had to looked before they could account for the plot details of later seasons. “Development Hell” is the wrong term, this is “Development Lead Time”. Post Season 4 things like Trixie and Starlight could be thrown in as cameos because they did not effect the already written story.
 
As for their ignoring existing allies: Twilight so obsessed with this she’s forgets the worth of her existing friends and her friends not taking this seriously is a major plot point.
 
And they didn’t know the Storm King overthrew them until afterward.

@Background Pony #6862  
If it was, how did Starlight and Trixie appear together during “We Got This Together”?

Sad but true.

@DaveMan1000
 
The movie was announced for 2017 and there was not even one delay. It was released in 2017 as planned. How is that development hell?

Yeah, you can tell that this movie went into production around Season 4

Development hell, it’s a great place.

@ObliviaTheChangeling
 

Also:
 
B U Y  
O U R  
T O Y S !

Yeah, pretty much