User blog comment:TheDoctor230/Crossovers/@comment-27700324-20170528165906/@comment-27700324-20170528195639

Well they can't exactly have both. Lots of plot and character development is going on in each season and we already have a bunch of crossovers on Flash's side as it is.

Supergirl didn't have a crossover cause they had plans already. Lots of stuff with Cadmus, Rhea, and just a lot. Plus the show was renewed later than the others, so they had to catch up with making the story of the show's second season.