Does the GTD include incidents of state terrorism? No, the GTD is limited to acts of non-state terrorism. One of the three necessary criteria for inclusion is that there must be sub-national perpetrators.

https://www.start.umd.edu/gtd-faqs#q3

I would assume this is why, as the prevailing theory is Lockerbie was sponsored by Libya

Wikipedia suggests the database isn't particularly academically sound either.

https://en.wikipedia.org/wiki/Global_Terrorism_Database