Assuming Fux's counterpoint was written in 1752, why are there some references to the modal system and none to tonality? I mean... 1752 is way beyond the end of modality! Composers already had the notion of tonality in 1700: or am I wrong? If not, why did Fux didn't reference tonality but just write about the modal system?
First, Gradus ad Parnassum was completed in 1725 (not 1752), so it's a bit earlier than you think, although still in the time frame when tonality was becoming common.
Second, Fux was intentionally looking back to earlier styles of music, explicitly the music of Palestrina (who died 1594), and was, in a sense, taking a historical view even when it first published. In his introduction he even indicates that Aloysious, the teacher in the dialog, is his surrogate for Palestrina.