Maybe it would take the impact away when the Other Wally shows up.
Maybe they didn’t want to confuse new readers with Two Wally West’s, since elder Wally was a plot point.
Maybe they wanted to focus on the Titans found family connection between elder Wally and Donna. But the real family connection between the two Wally’s with mess that up.
However, Impulse is right there so it ruins my theory.
Still, long term both Wally’s can coexist. Hell, look at Supergirl and Powergirl.