I'm not sure why the Cats were removed from MTG...
It's not that simple. They were never removed. They still exist in the game, but … But the truth is much darker than that …
Let me give a short history lesson:
At the creation of our world, at game tick 0, also called the Big MapBang, the world was fully composed in equal parts with rainbow matter and candy matter.
After only 2 ticks, the rainbow matter and candy matter starts to randomly chemicially react with each other and rapidly grow pop tarts and large sandwich bodies all over the place. It's a huge gooey and icky mess. It's so gross, even the grossest Gross Magazine of Grossy Grossiness would say it's way too gross to print. Let me share just one harmless fact with you: Sometimes, it just decomposes with a funny fart sound and turns to a block of air.
After ca. 500 ticks, only a couple of short stripes of rainbow/candy matter remains. At the ends, the pop tarts and candy bodies start to grow faces. The Two Races of the Ancients born, the Nyan Cats and the PB&J pups! Everyone knows the Nyan Cats, but have you heard of the PB&J Pups? Those were the Nyan Cat's companions, they wre puppies with a brown fur in a peanut butter & jelly sandwhich and they poop out candy all around.
A few ticks later, both races came to life and happily fly thru the universe, peacefully. They sing funny songs, fart out rainbow matter and candy and eat it again to grow and create offspring, play video games and watch funny cat videos and make up stories about legendary Ceiling Cat. The Ancients survived by eating what the other species pooped out. PB&J pups ate rainbows and Nyan Cats ate candy matter. It was a peaceful and simple life back then. Their life was happy but short.
Every time a nyan cat and PB&J pup crossed paths, a mese block appears. The word “MESE” is actually an abbreviation in the language of the Ancients. “ME” is short for “Meowus Encientus” (Nyan Cat) and “SE” is for “Sillinussus Encientus” (PB&J Pup). “Encientus”, of course, means “one of the Ancients”.
Then, after tick 1000, a terrible event happens: The Era of Mapgen begins! Large space crafts, called the “MapBlocks”, swarm the world, crushing nearly every being in the world. They are commanded by the evil Tac Nayns, the destroyer of worlds!
They work by simply heating their surroundings to such high temperature so that nothing but MESE can survive. But by their sick twisted sense of humor, they actually allow some of the Ancients to survive. Not because they are nice, however … They use advanced brain wave generators to hypnotize them and force them to wage a terrible war against each other, until nothing remains.
That's all I know about how our world was created. Nobody knows where the Tac Nayns went, but they say they are watching you … The Tac Nayns are masters of disguise. Some say the Tac Nayns were never gone and are just exhausted from their war and went to sleep on the moon until it surrounded the World for an unknown number of times. It might by 1 moon cycle, or 10 million cycles. Nobody knowns. Some other says they hide in random blocks in the world and every block might be a tac nayn in disguise, waiting to eat you. But no matter where they are, sooner or later, your time is over. They will again wage war over the world. And again. And again. They are plotting to kill you. Not now, but eventually …
But if you think the tac nayns are the most evil race in our world, think again … Every time you create a new world, you give birth to billions of nyan cats and PB&J pups … only to murder them shortly after. The tac nayns were merely your slaves. You monster.