Why don’t the police ever tranquilize suspects?
While I am not sure this has never occurred, especially in countries other than the US. I would estimate the main reason is because tranquilizers do not work fast enough and you need to be very close to deploy them. So safety is one. The second is those types of drugs can be an issue for people and how they might react to them. Sedation is concerning in many ways, just ask the doctors that do it daily for procedures. There are risks to all sorts of non lethal options, tazers, pepper spray, rubber bullets, bean bag rounds, all come with issues that surround their use.
There would need to be a lot more trials and testing before tranquilizers would be used in law enforcement as far as I could tell.