why in a p-n-p transistor holes are majority carriers through the transistor but electrons in the external circuit?

Holes are nothing but the free spaces in the valence band.

 

Actually in conduction band free conduction electrons moves and constitute electron current but hole current is nothing but the flow of valence band bound electrons through free spaces (called holes). So ultemately only the electrons ( conduction band or valence band) moves inside and outside the transistor.