Why Does the Veil Between the Worlds Exist?

In the beginning the Magic World and the human world were one. Magical beings coexisted aside non magical beings; beast of legends and myth aside beast of common domestic and wild. And for a while there was peace; for a while their coexistence was of a mutual benefit. Until the uprising came. It began with the stirrings of unrest between the Two Kinds: Magic Kind, and Nonmagic Kind. Each began to think the other inferior, and when one was thought to step out of line, the other retaliated viciously until minor battles were spawned from the blood hates that were born for the 'injustices' that were wrought. 
At last, far north in the lands where Celtae and the Sahson beliefs warred with one another, a man of great power, Merlin Emrys, appointed a new leader in hopes that a peace could be achieved among the Two Kinds. The new king he appointed was a boy named Arthur. A mere mortal who wielded a sword of great magic named Caledfwlch.
After Merlin appointed Arthur Pendragon and Caledfwlch as rulers, the sword and the boy reigned side by side for many long years, each teaching the other how to understand the opposite kind and by doing so creating peace throughout the land. A peace that lasted through the countless attempts to upstart war from those who did not want peace amongst the Two Kinds. It wasn't until Arthur's death by the hand of his inbred son, Mordred, who stood against the alliance of the Two Kinds that this peace was destroyed.
Arthur was loved by all in the kingdom, and his death caused a great ripple throughout the land. Fueled by the whisperings of Morgan Le Fae, each side began to blame the other until war was not only threatened, but instigated by an army of Magic Kind led by Mordred, against the Knights of the Round Table. She created a huge uprising as well. Though consisting of mixed kinds, the King Arthur’s Knights stood together with the forces of Camelot to hold off the oncoming army.
It was a slaughter.
The forces of Camaalot, set in place by both Merlin and the Sword Caledfwlch were far too great for Mordred’s untrained army of hot blooded soldiers to overcome. But though the forces of Mordred were driven back, those of the Nonmagic Kind were enraged and began to demand that all Magic Kind be driven out of the land and eradicated so that true peace could once again return. But those in the Knights of the Round Table who were of the Magic Kind, and had families therein felt wronged and torn between duty and family. For to them, their duty to their people was like the bond of family… but when they tried to reason with the people, they were called traitors and up starters, rebels and cowards, and driven out of Camelot.
The Knights were reformed, this time under the rule of Syr Cai, who chose only Nonmagic Kind, and those against all Magic Kind, to fight aside him. In the days that followed, chaos ensued. Dragons were hunted down to near extinction for sport; Fae were poisoned with iron; magical creatures hunted down for their medicinal properties as well as for sport; elves were driven from the ancient forests which were cut down to make way for progress; and humans with magical abilities were called wizards and witches, warlocks and enchantresses, sorcerers and sorceresses, then stoned, beaten, tortured, burned at the stake. The world began to turn against all things magic.
When the Magic Kind began to fear their extinction, Merlin came forward and gathered all to him in a secret place. At first, they blamed him for the situation they were in, angry at him for not for seeing and preventing the slaughter of all things Magic and Sacred for his was the gift of foresight. But Merlin spoke and said unto them he could bring them to a world where there would be no one to persecute them. Slowly, they began to listen to him, then in earnest demanded to know if such a place could really exist.
He told them it didn’t exist, but that it could be created.
When they demanded how, he told them of a spell that would create a rift between the world of the Magic Kind and the world of the Nonmagic Kind so that a wall – a veil – could be raised between the two of them and they exists both side by side and separately so that each of the Two Kinds could live in total peace. Some loved the idea, others were uncertain, and others still did not like the idea of leaving the world they had known for generations. Some refusing because they blamed him for the chaos that had ravaged their world. Merlin assured them that those who wanted to stay could, and those who wished to leave just need stay in the Place of Gathering until the spell was complete. It would take three days to complete the set up for the spell, so those who wished to think on it could decide. 
Those who did not wish to leave – the Seelie and Unseelie courts and most of the fae, the giants, and a few other creatures of magic – left in the days following, and by the end of the three days, all who would stay had gone, and all those who wished to go to this new world, were ready. When all were gathered, Merlin stood and spoke to the gatherers explaining to them the effects this spell would have on him, and everyone within the confines of the Place of Gathering.
Firstly, the spell would consume an enormous amount of energy, and the result might not only make the people very tired and drained of energy once it was through running its course, but could in fact use his own life force and power to complete it. Since he was a very powerful wizard – one of the most powerful in all of history – there would be no other loss of life than his own.
Secondly, he had chosen, in secret, six Magic Kind that would each inherit one portion of his power, so that should something happen and the veil began to weaken, or another crisis arose, these six would have the power to band together and overcome it.
When these Magic Kind had grown old and finally passed away, the power would then move on to the next generation, so that this protection would last through the ages. The new generation would not be born until the predecessor had passed. However, the power would not be passed through blood lines, but at random, through those who would have the capability of not only wielding such an awesome power, but the potential to be a great leader as well. In this way, no one family would be held above the other, nor corrupt bloodlines created for hierarchy; for the power was not given to raise one above the other, but to help all Magic Kind when they were in dire need.
And thirdly, once the spell began, those within the Place of Gathering would lose consciousness only to awaken in the new world where none of the Nonmagic Kind could enter. It was then he held up a satchel said to contain six knives made of bone, and six stones of magic that were to be given to the Magic Kind Di-Ne when they were old enough to use their powers and then passed down through each generation to the next power holder, then the next. The knives were said to be the gateways across the veil, and the stones, though Merlin didn’t give great details as to what they did, were rumored to be powerful shields. After Merlin entrusted the satchel to an apprentice, he began the incantation that created the veil and separated the two worlds.