Maybe, the total number of incidents rises just because with the spread of the Internet and other communications technologies, it’s easier to get information about terrorists attacks. For example, there definitely were terrorist attacks in USSR before 1989(https://en.wikipedia.org/wiki/Category:Terrorism_in_the_Soviet_Union), but they aren’t mentioned in GTD.
Maybe, the total number of incidents rises just because with the spread of the Internet and other communications technologies, it’s easier to get information about terrorists attacks. For example, there definitely were terrorist attacks in USSR before 1989(https://en.wikipedia.org/wiki/Category:Terrorism_in_the_Soviet_Union), but they aren’t mentioned in GTD.