Understandably, Halpern and Schumacker and the rest of the creative team didn’t want to tire audiences out on those characters beyond Harley, so the Suicide Squad took a backseat.
Amazingly enough, David Ayer’s Suicide Squad was still a pop-cultural talking point when the Harley Quinn TV show was coming together, which was also before DC Universe went wide with its existence.
That said, one can easily read into the distinction that they’re not in the first 26 episodes, since it lightly implies the villainous group might indeed get involved at some point after the 26 episodes have aired.