I know for most parts of the world it's because there was either no writing or only proto-writing, but for the Eastern Mediterranean, how come detailed knowledge of the historical record is basically limited to Egypt? We've deciphered Akkadian/Sumerian/Hittite cuneiform as well as linear b, so why is the historical record of this period still so rife with speculation and debate when compared to the classical period, or even the early iron age?
That is a mistaken impression, as we know at least as much about Babylonian and Hittite history as Egyptian history. Put a different way, you are underestimating our knowledge of Near Eastern history and overestimating our knowledge of Egyptian history. As the Egyptologist Alan Gardiner said in Egypt of the Pharaohs:
Even when full use has been made of the king-lists and of such subsidiary sources as have survived, the indispensable dynastic framework of Egyptian history shows lamentable gaps and many a doubtful attribution. If this be true of the skeleton, how much more is it of the flesh and blood with which we could wish it covered. Historical inscriptions of any considerable length are as rare as the isolated islets in an imperfectly charted ocean. The importance of many of the kings can be guessed at merely from the number of stelae or scarabs that bear their names. It must never be forgotten that we are dealing with a civilization thousands of years old and one of which only tiny remnants have survived. What is proudly advertised as Egyptian history is merely a collection of rags and tatters.
I recently wrote about the sources for Hittite history.