Why does a ferris wheel acclerate?

I have to assume that you are referring to the Rim of the wheel where the seats are attached. That being the case, the circular motion of the wheel's rim creates Centrifugal and Centripetal forces. These forces act against each other along the radius of the wheel, at every point on its circumference. Unless the wheel rim flies apart, the two forces are exactly in balance.

The acceleration is caused because the points on the rim are constantly changing direction as the wheel rotates. In order for a real object to change its direction of motion, a Force is needed.

Remember that Force = Mass x Acceleration. So an Acceleration is needed to generate the Force to change the direction of any point on the rotating wheel. That acceleration is felt by the riders of the wheel as being lifted off the seat, or pushed into the seat...depending on their position relative to the ground (force of Gravity), and the speed of the Wheel.