"Me too, Victor."
Translation:Мені теж, Віктор.
There's probably not any one real reason. Languages in general just tend to simplify over time. Even among other languages with a vocative case, it's often replaced with the nominative. So it could just be that.
Or it could also be due to Russian influence, but even in Russian a new ending seems to be forming to replace the original long lost vocative (specifically, the loss of the final vowel in nouns ending in -а).
I think it the course creators left it out to make it easier on learners. But the vocative isn't really that hard, and it's still encountered often enough to be useful. So maybe they might consider using it if/when they're able to make a new tree version.