Because they are the popularest letters.
A s d f g h j k l
The home row is the main section of a keyboard, it is the middle row of the keyboard.
It is the 2nd row of letters in the QWERTY keyboard
The home keys on your keyboard are the letters A S D Fand J K L.Home keys are used by touch typists who type quite fast and don't really look down at the keyboard to find the letters, they usually know where they are on the keyboard because there are raised lines or bumps on the keys F and J, these help them find their way back to the letters without having to look down.
The letters F,G,J,L,N,P,Q,R,Z have no line of symmetry in the verticle or horizontal.
A mix of letters only including J, D, G and F. Clearly done by someone who hits multiple keys on the keyboard or done by sliding your hand across the keyboard.
Letters without a line of symmetry are: Lowercase: a, b, d, e, f, g, h, j, p, q, r, and y Uppercase: F, G, J, L, P, Q, and R
The reasoning for the keyboard setup is that commonly used keys are not close together as not to cause confusion for the typist. Also, the first letter is a Q, and letters not commonly used are in the corners, edges, and sides. The most frequently used letters are colser to the KEYBOARD HOME ROW, which is the exact middle line of your keyboard (A S D F J K L and ;)
The bumps or nipples found on the F and J keys on the keyboard helps users correctly position their left and right hand on the keyboard without having to look at it. For example, without looking at the keyboard you can easily position your left index finger on the F and the right index finger on the J, which then positions all the remainder of your fingers in the correct position.
R,l,p,q,i,d,f,g,j,
It's "Home Row" on an American Keyboard. In other words when your hands are in standard typing position, your fingers will be resting on the asdfjkl; keys from left to right.