because i think it hinders people from joining the comp scene, the learning curve is already so damn steap its ridicoules. then before you can even start with that you gotta spend hours grinding inorder to breed a pokemon that you'll might soon realize isnt that good/doesnt fit the team/lacking eggmoves.
also you need more than 6 pokemon inorder to do comp, having a selection of pokemon is necessary.
yes you might loose a few battles if your pokemons arent 5x31. but you will loose way more battles because you dont have to knowledge about the meta/ability to predict moves, movesets, switches etc.
im speaking out of my own expirience aswell here, one of the first things i breed when the new breeding came out was a 5x31 marowak, which i have never used in battle because, the nature and EVs are pretty meh. so i wasted a week of playing and 7-800k on something useless