In addition to what @FinnishPokéFan92 said, the explosion is a fire one. Fire spreads, and Torterra's literally covered in flammable grass. Torterra will have a very hard time using the move.
ok then what about casting fire outside of it? or coming from the ground?
if it would only come from the tip of its rock on back and change into a beam or point rage explosion , some ecplosion have point range, and dont need to burn or hurt the user.