User blog comment:ClassicFan92/A Question about Supergirl/@comment-3423279-20161115023448

I think the reason we haven't is because unlike Constantine, no Supergirl characters have ever appeared on any of the Arrowverse shows, and only Flash has gone to their world once.

Plus, aside from Barry's brief question of "How long was I gone?" There has been literally no discussion about it by any of the characters.

Granted with the future Crossover this might change, but until then I don't think there is enough of a reason to do so for now.