Last year, DC Comics and the CW did something unprecedented, they combined their 4 superhero shows into one event called “Invasion”. This featured the characters of Supergirl, Flash, Arrow, and Legends of Tomorrow. However, many noted that the crossover felt light in the Supergirl area. For she kickstarted the event in the literal last minute of her show, and then was barely used the rest of the time.
When asked about this at Q&A (covered by IGN), executive producer Andrew Kreisberg noted that the real reason for this was that timing was against them. Not only were they making their own shows and the crossover, they didn’t know when they started the seasons that Supergirl was coming to the CW:
“We weren’t aware that we were going to switch Supergirl from CBS to CW, and it was really going to be part of [the crossover],” he said, going on to note that “because the [crossover] starts on Supergirl, Melissa [Benoist] ended up being heavily in Flash and then in Arrow. Supergirl was the show we needed to shut down the most, and it was the one we didn’t have any opportunity to.”
However, with all four shows being renewed, and the process of planning out next season already beginning, he states that the next 4-part crossover will be a true crossover.