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 degree
360
1
360 degrees
Exterior angles 360 degrees Interior angles 1260 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.
360 degree
360
Interior angles = 360 degrees Exterior angles = 360 degrees
360 degrees.
1
A circle is divided into 360° and each of them is 1° ■
360 degrees
360 degrees
A quadrilateral (4-sided polygon).