Well Heatran, as a legendary pokemon, is both rare (there may not even be more than one, in-universe of course), and in all likelihood immortal. This would both make breeding difficult, but unnecessary. From Game Freak's perspective, they would not want trainers to have easy access to multiple, powerful legendary pokemon in the game and would want to make each individual legendary all the more special, and as such would not want legendaries to breed. Hope this helps