Question

Why do we use only X and Y as coordinate axes why can't we use A,B,C,P,Q,R etc

Answer 1

You'll find details on this point (and precise references) in Cajori's History of mathematical notations, ¶340. He credits Descartes in his La Géometrie for the introduction of x, y and z (and more generally, usefully and interestingly, for the use of the first letters of the alphabet for known quantities and the last letters for the unknown quantities) He notes that Descartes used the notation considerably earlier: the book was published in 1637, yet in 1629 he was already using x as an unknown (although in the same place y is a known quantity...); also, he used the notation in manuscripts dated earlier than the book by years.

I read somewhere this convention was started by Rene Descartes. While conceptualizing the coordinationn system, he used x and y to denote the axes. It took root from there on and has been used ever since.