Coverage. For making things easier for beginners. Here's what I mean :
- Firstly, no longer any weakness to Rock-types
- Then, the amount of Rock types in-game( I mean, without repels, a Boldore is a constant menace/ nuisance)
- The 2nd and 7th Gym become much easier to defeat. Specially the 2nd gym.
- Moves like Brick Break/ Arm Thrust, which are generally strong get STAB (Same Type Attack Bonus), so they hit harder.
You see, if one is playing a Pokemon game for the first time, then defeating gyms becomes a challenge. So to help such beginners, Gamefreak made it that way. Fighting is one of the best offensive types, and hence it suits the style of those who go for the kill, i.e. focus on Offence, rather than Defense.