I'm inclined to agree with @Giorgio-Chiodi, I'd like to avoid another iMessage fiasco by constantly pinging users when they've only replied once to a topic. It's fine how it is now.
I post in a lot of topics, doesn't mean I want constant notifications when someone else does. If I'm interested in a topic, I'll watch it, if someone is directly talking to me, I'm already notified. If someone is talking to someone else, I don't need to be notified of this, as it's not in direct relation to my experience on the board.
I'm aware Facebook does this with photos and comments, but it annoys me to the point where I delete my comment just to stop receiving the notifications. Just because I've commented on something, doesn't mean I want notifying of everything that happens thereafter, unless they're directly talking to me.