Why Scrum is alive and it wasn’t replaced by XP?
The answer is very easy, as easy as understanding Scrum. That is it. Scrum is very easy to understand to everyone and there is no coding in there that can confuse the bosses. XP, if you check the official webpage and their diagrams, well, you gotta spend some time to get used to that huge graph (and to apply it, quite a bit more). Nevertheless, it is basically detailing how to carry out the agile principles, when it comes to technical stuffs. I read papers saying that XP doesn’t have planning. Well, that is not true. Of course you have planning in XP, such as week iteration, or which of the functionalities should be implemented. Basically XP tried to give an step and clarify a bit the until-that-time dark part of what to do with the technical stuffs. The reason why XP didn’t replace Scrum, is just based on the incredibly good terminology and clarity of Scrum itself. Made for business people and not programmers. That is why, the combination of Scrum (for management) and XP (for programmers) creates a really good combo, and it is widely used. I recommend you to forget about FDD. It is just a trial to import traditional to agile, and it is not agile in my opinion because it leads to a traditional approach with the time.
Frameworks such as Scrum or XP are good because they can sustain an agile philosophy through the problems. FDD with the time (and I lived this in my own flesh) deteriorates easily.