That's because, as a healer, you're dependent on your team not being bad and ignorant as to the presence of an actual healer (I suppose they don't get to see many, regardless of the faction you're playing - because it's always the OTHER team that has the healers and never your own, you know
). So, since people generally don't protect their healers, you can't make much of an impact, you can't help your team, your team is effectively down one person.
Seriously, as a healer, you should always bring a friend and prioritize healing them above all others. Because your friend will try to keep you safe. Your average random guy in a BG won't - though there are notable exceptions <3