When and why was it decided that the 'low' end of a keyboard instrument should be on the right and the 'high' end on the left?

probably evolved that way because early lyre type instruments would have been held in the left hand and played with the right. as these evolved into guitars and keyboards, the playing/plucking would still be done with the dominant hand. the accompanimental stuff which is usually lower would have been put in the non-dominant hand.

today, music can be just as difficult for both hands, and melodies can show up in any voice... but starting out, it was logical to have the lead in the hand that is dominant for most players.

(I'm assuming u switched low and high by accident)

The low end is on the left not the right.

The low end of a piano keyboard is on the left and the high end on the right because most people are right handed and the melody is usually played on the high notes

You got that butt-backwards, kid