Here's why:

* Shape: The Arecibo telescope's main dish is a large, spherical bowl.

* Focus: Unlike a parabolic reflector (which focuses all incoming parallel rays to a single point), a spherical reflector has a focal curve. This means the point where the signals are collected (the "focus") is not a single point, but a curved line.

* Design: The Arecibo telescope uses a secondary reflector (a smaller parabolic dish) located at the focal curve of the main dish. This secondary reflector reflects the incoming radio waves to a single point, where the receiver is placed.

While the spherical design is not as efficient as a parabolic design, it allows for a much larger and more cost-effective construction for a radio telescope.