Imperial collapse. For the Egyptians the loss of empire to invasion and then to the collapse of the Western Roman Empire would do it. Also written Greek was both the official language of their conquerors and a more efficient script. Europe came very close to losing writing when Rome dropped the ball. It was saved, I hypothesise, by the fact that the Alphabet is such an efficient system. All of which makes the accomplishments of the Chinese more spectacular.

I agree with @Jnei.

You can say the same thing about the English. Just look at Olde English and modern English. Very, very different.

Assimilation of the population? Demographics change.

The Rosetta Stone features two ancient languages - Ancient Egyptian in hieroglyphs and Demotic script, and Ancient Greek - with no modern languages appearing on it (it'd be surprising if it did, since the inscriptions were created in 196BCE).

Languages are sometimes abandoned, however. In most cases, this is because a culture is invaded by another, or simply overwhelmed by a larger or more dominant culture - as was the case with Native American languages, Celtic languages such as Welsh, Cornish and Breton, and various others. Fortunately, many of the languages that looked set to vanish over the last few centuries have been preserved and in some cases have even made a comeback - it's not uncommon to hear Welsh spoken in Wales now, whereas 50 years ago it had almost died out.

time and illiteracy.