We don't know that the cartels don't carry out terrorist attacks, but i don't think they're interested in not having these things in their own country. They're not like Islamic terrorism, which is much more infamous. They were having broader business problems, so i don't think the approach will be the same, it will be very different. That's why they believe there aren't any problems.
What do you mean we don't know? Usually when there is a terrorist attack sooner or later we know who was behind him, many times it's the same organization that announces it because they are "proud" of what they did. Mexican cartels don't operate this way. Even in the areas with heavy touristic traffic is rare for a tourist to become a target unless he is involved in some shady business. You can just do a simple research online and see if you find any terrorist attacks performed by the Mexican cartels. I don't think you'll find much.
To be fair when it's USA (or Mexico in this case) there are MANY terrorists organizations around the world that hates them. You do not get to hear anyone that hates... Spain for example or anyone like that, because these nations don't really do anything to the world, but when it's USA or nearby, it's dangerous because we get to see them actually do go out and hurt other nations so there are many who wants a revenge.
I mean not that cartels would do anything, because any attack on world cup would mean cartels would be done completely, you would see swats and navies all go in at the same time to stop them completely as a response. There is a reason why cartels try not to kill DEA agents, nobody wants that much heat. But terrorists do not care, so lets hope there is none.