Because Game Freak want them to be Megas!
I respect that there are some Pokemon that deserve a Mega, but Game Freak makes what they want and some new Megas are to improve their stats as nobody uses them.
Audino and Slowbro aren't good Pokemons in my opinion. Game Freak maybe saw that too, and decided to make a Mega for them in order for them to become more used by the players.
Now the Megas;
Mega Slowbro remains Water/Psychic type but has the ability Shell Armor.
Mega Audino is now Normal/Fairy type and keeps the ability Healer.
Slowmbro's Shell Armor prevents from it receiving a critical hit. With the great defenses, it's now a great bulky Pokémon.
Audino is now Fairy-Type making it with more weaknesses and ability Healer making it receive 30% HP when switch from the battle. Audino was great HP / Defense / Sp. Defense and if they improve a little more, this can become the next Alomomola.
I hope there will be a Mega for Hydreigon!
Hope this helps.