It was an average day in the Everfree forest, The sun wasn't shining, flying abominations were singing, and various monstrosities were out and about, going about their business; it was perfect! Suddenly, however, a terrified timberwolf ran through the dense, green, undergrowth, catching the attention of several other monsters.
"It was horrible!" the timberwolf cried, "It was the most terrifying thing I've ever witnessed!"
"What was?" asked a manticore.
"I went outside the Everfree forest, and it was so bright and sunny out!" said the timberwolf.
"Sunny!? I...hate the sun!" said Slendermane, as he walked out of the shadows, "It's so bright...and frighteningly cheerful!
"But, that's not even the worst part about it!" said the timberwolf. Panicking, the timberwolf elaborated ,
"I saw the most horrifying creatures there! I saw things that defied all description! Let me tell you about them..." a nearby manticore shook his head in frustration at the timberwolf's contradictory statement.
"There were things that looked like Slendermane, except shorter, and they had faces." There was a collective gasp, and a nearby cockatrice fainted. The timberwolf continued, "These hideous beasts came in all kinds of grotesque colors, and they all had these bizarre pictures on their flanks!" The manticore, now visibly shaken, decided to ask a question,
"So what did you do?"
"I just stood there. I wanted to run, I wanted to flee for my life, screaming, but I was to afraid. My body wouldn't obey my wishes, I was stuck there, with those...those hideous things!"
"I take it that you regained control of your body then, and ran back here after that?" a poison joke plant said, but the timberwolf just shook his head, with a haunted look in his eyes.
"No, I didn't," the timberwolf confessed, "I hadn't yet summoned up the courage to do so. That is when it started getting even worse..."
"What do you mean worse?!" asked a cragodile, "What could be worse than that?!" the timberwolf just swallowed dryly,
"After that they started doing something I thought I would only ever witness in my worst nightmares: They started singing!" There were several screams.
"No! That can't be...It just can't!" stuttered a sleeping gas plant.
"But is is! After that they started dancing!" the timberwolf whimpered.
"Are...are you certain?" a star spider asked, shaking, "You may have just saw something else that you mistook for dancing..."
"No," said the timberwolf, "I know what I saw! There is no denying they were dancing!" the timberwolf half-shouted. The timberwolf then regained his composure and continued with the tale of his ordeal,
"Then, the truly unthinkable happened..." the timberwolf looked got a far away look in his eyes, "a pink mare came out and said something that chilled my very soul. She said "Lets throw a party!" There was nothing but silence in the forest then, as all of the assembled creatures stood shocked beyond belief at the use of such a sickening word.
"We have to tell everyone else about this!" said the cragadile, "Everyone else must know that it is not safe to exit the Everfree forest!" the other monsters all nodded in agreement.
About two weeks later, everyone in the Everfree forest had heard about what the timberwolf had witnesses, and the entire forest was in a state of panic.
"What do we do now?!" asked a concerned chupacabra.
"How will we protect ourselves from those happy, colorful abominations?" inquired a vamperic jackalope.
"Maybe we should all barricade our homes!" suggested an ursa major. After a while all the creatures of the Everfree forest were brainstorming to figure out how to keep the dreaded ponies from entering their for lands. After multiple failed attempts to barricade their forest from the outside world, the creature began to realize that there was no real way to separate their beloved Everfree forest from Equestria. With few other choices, the various creatures decided that they simply must try to stay well within the forests boundaries, or risk the ultimate terror: Happy pastel ponies in a beautiful land of magical friendship.
So the various monsters gravitated towards the center of the Everfree forest. Many other monsters, deciding not to believe the stories, decided to venture out into the main part of Equestria, but quickly came rushing back in again, often bringing with them similar tales of terror.
Eventually almost no monsters were found within the outer areas of the forest, let alone outside it. The only exceptions being the creatures that got lost, or those rare few who were naive and had somehow not heard the reports.
This is the real reason why everything inside the Everfree forest seem so hostile, and attack ponies on sight. This is also the reason why ponies only seem to encounter these monsters only in the deepest parts of the Everfree forest, and why the monsters seem so content to not try to invade other areas of Equestria.
It's not just because the Everfree forest is their home, or simply because of the fact that they do not favor the outside environment, (which they most certainly don't). No, because while those are all valid reasons, they are most certainly not the main reasons.
The main reasons the monsters stay in the forest, are because they are truly afraid; the various monstrosities are loath to leave the perceived safety of the Everfree, which is their only protection from the outside. The monsters fear the overwhelming happiness of the rest of Equestria, and are horrified by its adorable inhabitants. While the assortment of monsters may hate sunlight, it is not the worst thing, while they may have a very real disdain of bright, cheerful colors, the colors do not truly frighten them.
It is not a magical spell that keeps the monsters in the Everfree forest, but fear of the ultimate nightmare. Creatures of practically unfathomable horror, capable of sending terror into the very being of the native species of the Everfree forest.
Indeed, it is not just the monsters that keep the ponies out of the Everfree forest, nearly as much as it is the ponies who keep the monsters in.
