Why Logic families assign voltage levels to the two binary states, and the assignment is a convention, not a law of physics. Positive logic is the common convention where the more positive (higher) voltage level stands for logic 1 or true, and the lower voltage level stands for logic 0 or false. Negative logic is the opposite convention, where the low or more negative level represents 1. TTL and standard CMOS parts are described using positive logic.
Watch out The choice about noise immunity describes a different property entirely, how much interference a family tolerates before a level is misread, which CMOS does well but which has nothing to do with the logic polarity convention.
Positive logic: higher voltage = 1. Think "high is true." Negative logic flips it.