Elementary, my dear Watson
There are several modes and artificial scales that only change one note in a major scale, and this is one of them is the most basic and most frequently used.
To make simple minor, “simply” lower 3. The simplest example is C simple minor, which simply flats E:
C D Eb F G A B C, or in any key:
1 2 b3 4 5 6 maj7 8
“Simple minor” is my name because it is descriptive.