There are renewal charges and having an license is costly task and there are lot of other implications that needs to be fulfilled but most of the casino first of all go for license as players search for the details before joining any casino like us.So at this time Curacao license is the most famous and somewhat cheap at this time so you will find most of the casino operating under the same license.But after that they can turn out to be scam also so make sure you play over legit casinos only.
A license gives more credibility to the site but it is absolutely no guarantee that they will not scam you. It also works in the other way: an unlicensed casino can still have a very good reputation, but it takes some time to build reputation. I am always carefull with newly launched casinos even when they have a license.