Once upon a time, I was Defender of the Prologue! I was thinking about getting a cape and a badge and everything. I didn't understand the war on prologues at all. I'd open a book, and go, "LOOK! THIS BOOK GETS A PROLOGUE! WHY CAN'T I HAVE ONE??* Yet, everywhere I looked, agents would say the same thing: Delete the prologue, or worse, they don't even read the prologue when a writer sends them pages. So if you sent a prologue, you basically lost your chance to have that agent read pages. And I didn't understand why it was so bad to set up the story before Chapter 1.
But now, I have seen the light! Over the past week or so, I had the pleasure of reading approximately 160 first pages entered into NestPitch, and approximately 150 first pages entered in PitchSlam. There was some overlap, but I'd wager I still read about 200 distinct, separate first pages. And way, way too many of them had prologues. Most of the prologues I saw did not do what the authors wanted us to do. Nearly all of them confused me, and with a lot of the fantasy/sci-fi entries, if all I saw was your prologue, honestly, those stories are all running together in my head. You didn't make it stand out - you made your story forgettable by not giving me a first page to hold onto.
The purpose of the first page is to drop us into the here and now of your story. We've already read a little blurb about your story (be in the back cover, the pitch, or a query letter), and THAT is the story that inspired us to want to read your first page. When we get to the first page and it's set ten years ago, or worse, it's a different character entirely, we get confused. You don't want to confuse (or worse, annoy) you reader on the first page. There is a time and a place for backstory, but it's not at the very beginning of your manuscript. If you legitimately can't find anywhere else to put your backstory, find a beta reader or critique partner who can help.
The other thing prologues often do is drop us at the end of in the middle of the story, then back up to the beginning. DON'T DO THAT. It's unnecessary and redundant. Just let me see the beginning of your story. If the beginning of the story isn't the best place to start, then rethink the beginning. Reconsider your story structure, or find a different starting point. But there's no need to start with your main character addressing the reader. "Let me tell you, Reader, what happened to bring me to the point I'm at right now...." It's redundant.
And the worst thing about your prologue is that I can't evaluate the things that go on your first page - introduction to the character, intriguing plot, grounding in the scene, emotion, etc., if your prologue is in the way. Calling it Chapter Zero or Chapter 1/2 or "The Beginning" or even "Chapter 1" doesn't disguise what it is. We can still tell when we're reading a prologue (and I personally prefer that you be honest instead of trying to hide it).
Once you've sold about a million books, you can write all the prologues you want. But when you're entering contests or querying, seriously reconsider.
* Reality Summer had a prologue until it was universally rejected by 100% of agents. Requests started rolling in once I deleted it. My later manuscripts didn't have or need a prologue, and my current WIP has two. But, in my defense, I'm still on the first draft.