You are responsible for the safety equipment in your home and should you or your family burn to death because you didn’t fit sprinklers, you are responsible. In an apartment block, the owner(s) including local councils have a duty of care to ensure their premises are as safe as possible, and they can be held legally responsible if they don’t do that. That’s one difference. I’d say cost comes into it too. Individuals would not pay for sprinklers to be installed and, especially in your country, some would start screaming about government interference. Also, how would you ensure already built homes installed sprinklers? It would be a huge, huge task. It’s not even law here to have a smoke detector installed.