Because the lives fall back to B And B is always a much more difficult point to take. It usually comes with high ground that is inside and not exposed to an even higher ground such as the walls most A points are exposed to. B is usually a short walk from any point of spawn, be it A or transports and given how hard it is to crack B dying as an attacker is counter productive. Even if you run in and take one with you they spawn right back at the defensive line while you need to walk back to the point. This leads to a lot of conservative play as an attacker, meaning it's an uphill battle all the way. Now factor in the fact that if a gate is broken an extra point's worth of lives heads back there and it makes an assault near impossible save for enemy incompetence.