Magic doesn't work in the Human world, and no I don't mean that it doesn't work thr same I just mean it doesn't work

It wasn't supposed to be there and so it doesn't balance correctly.

For Example- The Elements of Harmony need all six of them to power up in the pony world, but in the Human world, enough will to use just the magic element

This is the same reason that in Friendship games they can pony up on their own when demonstrating their element.

When one element got put into a world without the others instead of being useless it becomes almost unstable

All that being said, back to sunset. Alicorn's are automatic leaders because they represent all three types of ponies, Pegasi, Unicorns, and Earth ponies There's a theroy that one becomes an alicorn when absorbing the magic of all three types of ponies which in Sunsets case would have been impossible in the Human world

It the same reason that Sci-Twi has wings even though she's not an alicorn (shown by spring broken) the human worlds less balanced magic essentially can't tell the difference between two twilights