Ok, but im not asking for all Sunset backstory in the movie i would expect some part. And there could been the place if instead to waste time for some fanservice stuff (You know, all the references) and other useless scene they could make the time.
In the movie the only backstory we get is that she was a Celestia's Students and went rouge, they didn't really bother to give us much and in the end this made her less interesting of what the settings already have.
OK. Question for you, then. What did they show in the comic book that they didn't include in the movie, but should have?
You mention a flashback later in your post. But what could have made this flashback actually give more information in a certain amount of time that Celestia didn't reveal about her in the movie? Because I'm not entirely sure if making this 8 page comic an entire flashback in the movie would have been good timing-wise.
He in the movie is nicely explain half of the story but it is not explained well, but is not like they said "Ah, Pitch, he is the nightmare king and so he likes to do evil stuff. Go get him guardians!", and still they made of him one of the best villain of Dreamworks.
Except that they didn't actually pull that half-assed description in Equestria Girls either. Celestia explains pretty well in what little time she has that:
-Sunset Shimmer was a former student of her's - her personal student, actually, before Twilight came along
-She did not get what she wanted out of studying with Celestia
-Because she did not get what she wanted, she became cruel and dishonest (meaning that she probably had ulteriar motives this whole time)
-She decided to pursue her own path in life, and she went into the mirror
-Celestia hoped that Sunset Shimmer would use the mirror to return and change her ways, but that never happened.
-Later in the movie, we find out that Sunset Shimmer never had any friends - and when she was transformed by the Elements of Harmony, she said that she had no idea how to make friends because all she knows is how to be mean to people.
Celestia never said, "Herp, she's evil and stuff!" While the comic expands on Sunset Shimmer's reasons for going into the mirror, it does not give any NEW reasons for it. Anything that Celestia didn't directly mention was either mentioned by Sunset Shimmer or heavily implied.
I actually found that the comic gave more backstory to Celestia than to Sunset Shimmer. I mean sure, it's nice to see exactly what happened to her in canon territory, but given the information that we got in the movie, anyone with decent talent could have written a fanfic that came to the same conflict and conclusion, just in different ways.
If they focused more on Sunset insted to inserting scenes like "The all-powerful and magic Trixie wants a snack-bar" or the CMC scene in the library, wich didn't serve any pruprose at all, there would have been the space for inserting a good flashback.
Those two "fanservice" scenes were not their own separate scenes. They were add-ons to scenes already in place, and each add-on took 30 seconds or less to show on screen. And those were the longest ones in the movie, so taking out all of them would not have made much difference.
I also explain in my most recent post (besides this one) why the movie had to be 70-some minutes long. I am pretty sure, too, that if they had cut the references/fanservice, they wouldn't have used that time to add more Sunset Shimmer backstory.