Basically because it was decided that way, there are plenty of other numbers that could have been used. With that said, 360 is a good choice, it divides evenly into lot of fractions and it has a resolution that is just about what's need for many applications.
360
360 degree
1
360 degrees
The 4 interior angles add up to 360 degrees
360
4 sides requires Total interior angles of 360 degrees 360-90-90-91= 89
The sum of the interior angles is 360 degrees.
Interior angles = 360 degrees Exterior angles = 360 degrees
360
360 degree
360 degrees.
1
360 degrees
360 degrees
A circle is divided into 360° and each of them is 1° ■
A quadrilateral (4-sided polygon).