I think it has more to do with culture than police effectiveness as we are not so popular as a country with good enforcement of law. Croatia is a small country and there is always someone that knows someone even if he is on the other side of the country. So even if somebody steals something from someone, there is a big chance that it is someone you know (he knows when you leave your house, when you get back etc.), so it would be a really big shame if you find out that someone you know stole something from you. Therefore, most thefts are done by Roma people because they are somewhat excluded/isolated from society.

In countries like Croatia the police is busy catching criminals instead of helping them inject heroin, going after mean people on Twitter, or calculating if the robbed amount is under 900 USD and then apologising to the thief and giving him a ride home like the SF police do.