Why are asdfjkl called the home row keys?

What you are referring to is the QWERTY (start of top left row) keyboard layout.
Rumor has it that this selection of letters were placed in order to slow down the typing speed. Why would they want to slow them down? Because, on the original typewriters, ink was transferred to paper via an arm. On the end of that arm was the letter you wanted to type. Some people were so fast that arms tended to get jammed and you had to stop and unclog them before doing anymore typing. Also, it it not a coincidence that the letter from the top row can be rearranged to spell "typewriter".