True, but in countries where tourism is heavily skewed towards organised groups, operators make the decisions rather than individual travellers. If itineraries to a country cannot attract sufficient business at an acceptable price the best course of action open to operators can be to withdraw the programme or switch-sell customers to alternative destinations. Tour organisers must also take liability into consideration. Tunisia is one of those countries which relies heavily on the goodwill of tour operators for a large proportion of its visitors.
Many visitors to London, Paris, Madrid, NY, Sydney etc. make individual travel arrangements. In destinations such as Tunisia where the decisions are often made by large tour operators, customers (however confident) will simply be informed that their preferred trip has been cancelled for security reasons. Would you like to be rebooked to (alternative destination) or receive a full refund?
Time is also a great healer. Tourism from Australia to Bali dipped sharply in the wake of the atrocity there, but numbers have recovered since. Likewise, most people have forgotten the attacks on foreign tourists in Djerba (Tunisia) many years ago but will probably be reminded of them tonight. Recovery can take many seasons. Kenya, which you mention on your list, hasn't come close to visitor levels seen when the major UK tour operators ran large charter programmes to Mombasa. They won't be back there in volume any time soon.