Why weren't there Islamic terror attacks in the 20th century pre-1979?
It sounds like you're referring to wars fought with Islam as the primary motivator, which really isn't that special, let's be real. Holy wars of all kind have been fought and it's generally understood that practising a religion and fighting a war under its banner are not necessarily the same- rather, they provide a motivation for "soldiers" a lot of the time, whereas the real intentions are politically-based rather than religious.
It sounds like you're referring to wars fought with Islam as the primary motivator, which really isn't that special, let's be real. Holy wars of all kind have been fought and it's generally understood that practising a religion and fighting a war under its banner are not necessarily the same- rather, they provide a motivation for "soldiers" a lot of the time, whereas the real intentions are politically-based rather than religious.