Sumerians had a durable form of written language that survived archaeologically significant timespans, while Teotihuacan did not (though some record of Teo activity is, interestingly, preserved in contemporaneous Maya writings and stelae, also at Copan which was for some time a Teo client/vassal/subject state).

This does not mean Teo did not have a written language. Only that the media on which such a language may or may not have been recorded was not one that survived the depredations of the monsoonal climate of the Teotihuacan valley, or (more speculatively) that some religious or political proscription on written text suppressed the preservation of such records.

That said, some recent and intriguing discoveries have been made in the Teotihuacan ceremonial core which strongly suggest that a substantial subculture of Maya (or Maya-trained) artisans and scribes were established in Teo at the time of its apogee, and may have written more about the city than we currently are aware, but this comes from extremely recent and presently inconclusive archaeological findings and is a lively and cutting-edge research area.