It's not that it's better, it's just that the rears are further away from the receiver so there's more cost involved. Then consider the rears aren't used as much as the fronts so I chose to use a cheaper thinner cable. I could also hide it under the carpet easier next to the skirting board.
Decide on your budget, determine how much you need and then shop around. I usually buy from eBay and Amazon but stores like RS will carry it too.