Why aren't the letters on a computer keyboard arranged in alphabetical order?

The basic answer to this is that the QWERTY keyboard
layout was devised because of the typewriter technology of the time.  This layout was
meant to prevent the typewriter's "typebars" from jamming
together.

Old time typewriters were made with each letter
on the end of a little arm that would fly up when you hit the key.  The arm would fly up
and hit an inked ribbon, making an impression of the letter on the paper.  If two keys
right next to each other were pressed at the same time, the typebars would both fly up
and stick together.

The QWERTY layout was adopted so that
letters that were commonly used would be far apart on the keyboard.  That way there
would be less chance of two adjacent keys being pressed at the same time and causing a
jam.