Short answer: Because that's what those copying ancient histories in the Byzantine east during the Middle Ages were interested in (though many works were probably lost before then). Even many of the books of Diodorus only survive in Epitome, meaning we only have Byzantine summaries of them.
To give you another important author: Polybius is the only historian writing during the Hellenistic period whose work survives in any sizable amount. He wrote about the period from the outbreak of the first Punic War (c. 264 B.C.) to the end of the Achaean and Third Punic Wars (146 B.C.). Much of his work only survives in fragments and summaries, but he is still one of our best sources for the second century in particular (both for the Roman and Hellenistic worlds) and was an active participant in many events he describes. He also references many other Hellenistic historians and quotes their work, though usually with the intention of criticizing them.
Then there's lots of other smaller works. There's 1 Maccabees and 2 Maccabees, which describe the revolt of the Judeans against Antiochus the IV. As the Romans become more heavily involved in the Hellenistic world, more and more of our information comes from them, whether books 31-45 of Livy or Appian's 2nd century A.D. history of Rome's foreign conquests (written in Greek) or the speeches and letters of Cicero. And then there are tons of inscriptions, set up by cities, Hellenistic kings and many others. You can find a good collection of them here:
http://www.amazon.com/The-Hellenistic-Period-Translation-Sourcebooks-ebook/dp/B000VYSSHC
If you're looking for a decent modern narrative and analysis of the Hellenistic period, this book is a good place to start: