This is a real "why" question... even though we know a lot of detail about the what and how of this, no one can say for sure "why" GTP is the favored source. We do know that many of the proteins involved in protein synthesis have structures similar to proteins that use GTP in signal transduction. There may be some energetics arguments regarding "why GTP" as well, but I am not personally aware of those arguments. None of my standard resources here provide reference to this but if I come across something, I'll pass it along.