Traditional publishers get thousands of manuscripts every year, and they publish let’s say 20 to 100 titles per year on average. So usually what you’ll get from a traditional publisher is a rejection. That’s not necessarily because your manuscript sucks. That’s just how it works with traditional publishers.

There are many factors that cause rejections, like:

Maybe your manuscript is boring, not interesting, too complicated, too basic, you name it.
Maybe you thought submitting your manuscript with the complete illustrations was a brilliant idea. No, you’re not making the publisher’s work easier by handling the illustrations. That’s their job.
Maybe the particular publishing house you sent your manuscript to isn’t a fan of fairy tales and you didn’t know that.
Maybe a publisher doesn’t want to compete with their own titles. Let’s say you sent a pirate story to a publisher, and they are working on another pirate story, or they just published one. Why would they publish a second one in the same period?
Maybe there was a huge market for grandparent-related children’s books a couple of years ago. But maybe now that niche is totally filled and nobody wants to see another one around for a while.
Maybe they think your manuscript is not profitable. Most of them follow “P&L”. At the end of the day, they are running a business, and your book is a product they have to be able to sell.
Maybe their sales team thinks you won’t be able to promote your book. There was a time when all marketing was handled by publishers, not authors, but that’s not the case anymore for many publishers. If they think you are not going to be good with book launches, school visits, social media, and all the other stuff for marketing, this may affect their decision.
Maybe the publisher is in a financial crisis and they think it’s better not to publish a new title unless you are offering them Harry Potter for free.
Maybe the publishing house you sent your manuscript has recently been bought by one of the Big 5. Maybe they decided to cut out new titles, or change their brand, or cut the budget in half, or they just plan to fire everyone and close the house.
Maybe your name, genre, age, or ethnical background isn’t what they are looking for. Have a look at this post. Catherine says she has 8 times more response from agents when she use a fake male name, with the exact same manuscript.
Maybe the editor personally doesn’t like you.
Maybe the editor was having a bad day and you’re just unlucky.
Maybe you thought it would be funny if you sent your manuscript in a box full of little shiny stars to explode on the editor’s desk when she opened the box. And maybe she is not a fan of that idea and didn’t even read it just because you messed up her desk.
Maybe the editor is so tired from all the other projects that he just didn’t get your genius joke about the character, and never read the second page.
Maybe the editor loved your manuscript and wanted to give it a go, but there was a secret office war between the editor and the sales guy, or another editor, or the publisher, or the marketing team, you name it. And just because the editor wanted it so badly, other guy wanted you out. Just to screw with the editor, nothing to do with you.

I am not making up these “office wars” or “editor’s bad day” things. I worked for a publishing house for four years as an art director, and I was one of the guys who decided which books to publish. I know it sounds heartless, but in a traditional publishing house, you have lots of things to do. And hundreds (if not thousands) of manuscripts to read. If you try to read all of them, you basically can’t do anything other than read manuscripts. What about your other responsibilities? What about ongoing projects and deadlines?

So, you must find a way to eliminate some. Everyone has their own ways. I never read more than one page, if I didn’t like the first page. I never looked at all the illustrations if I saw a couple crappy ones. I never read anything written in Comic Sans font just not to burn my eyes. I never read any manuscripts if the author was too pushy, repeatedly asking, “Have you checked it yet?”

I am sure there were many good manuscripts I never totally read, but rejected anyway. Even though I’m aware of that, I would do the same again. It’s how a traditional publishing house works. You have to find the bestseller in thousands of non-bestsellers. And it’s not even your main job to do.

In short, getting rejections doesn’t mean you wrote a crappy manuscript. I am not saying you wrote the most genius one. What I am saying is that it’s possible you wrote a genius one, but it ended up in a trash can of a publisher all the same. Jack London got 600 rejections in his lifetime. Lord of the Flies got 20 rejections. Dune had 23. Can you believe 23 different publishers think it wasn’t worth it to publish one of the world’s best science-fiction novels? Animal Farm by George Orwell was rejected too, for this reason: “There is no market for animal stories in the USA at the moment.” The last thing Animal Farm is about is animals; obviously, the editor didn’t read it at all.

Bonus: Have a look at J.K. Rowling’s tweet below:

.@babymegs_ Loads! First publisher to turn down Harry also sent @RGalbraith his rudest rejection. They don't even want me in a beard.

— J.K. Rowling (@jk_rowling) March 23, 2015

The good thing is, we are not in the sixteenth century anymore. You can publish your book by yourself. Or you can keep trying with traditional publishers. Just don’t give up by your twentieth rejection. When you doubt yourself, think about Jack London’s 600th rejection. It means he actually tried to get published at least 600 times. Maybe he would have gone with independent publishing if he had been working in our era–you never know.