Explain why there is no higher latitude other than 90 degrees north and south.

Latitude was designed to create a way to pinpoint location on the earth. We have 90 degrees of latitude north and 90 degrees of latitude south. What the degrees are measuring is the angle from the center of the Earth. The equator, which is the center of the Earth, is used as the starting point for measuring latitude. Thus, when we draw a line from the equator to the center of the Earth and then a line from the center of the Earth up to the North Pole, we have an angle of 90 degrees that has been created. The North Pole and the South Pole are the extreme ends of the Earth, and, as a result, you cannot have any angle greater than 90 degrees. Please see the attached image for a visual example demonstrating what I have described.
http://www.physicalgeography.net/fundamentals/2b.html