In Flash S02E18 "Versus Zoom", Flash accidentally passes through a blue portal during his speed test and after his return he asks how much time has passed. It's surely a nod to the crossover episode, but why did Barry not mention the new Earth he recently discovered? Why didn’t he mention his experience or Supergirl at all? Is there some behind the scene reason for it? Like channel issues or a directorial decision?
From the word of god himself, executive producer Andrew Kreisberg, god's employer said so:
DEADLINE: Even though this is a crossover that occurs mainly on the CBS, are we going to see any reference to it on The Flash?
KREISBERG: Yes, you’ll actually see the moment when Barry went on this experience and when he came back, for sure, in Tuesday’s episode on the CW — which is exciting. We had to flip because of when the shows are scheduled but otherwise the effect that it has on Flash I think is going to be minimal because that was what we were asked to do. We were just happy to have The Flash be on Supergirl.
DEADLINE: Well, the CW makes an appearance of sarcastic sorts on CBS and Supergirl tonight, with the “You look like the attractive yet non-threatening racially diverse cast of a CW show” line whipped off by Calista Flockhart’s Cat Grant looking at Melissa, Grant, Jeremy Jordan and Mehcad Brooks.
KREISBERG: That was actually one of the first lines I came up for this episode. What’s fun about the crossover is just because of the fact that you’re crossing over between two different TV shows, these episodes tend to feel a bit more heightened and you can tend to get a little bit more meta with them. Because when you put two TV shows together it does become a little bit of a meta-commentary on the differences between the shows.
The studios, and their meddling executives, have the final word, not the show runners (Pun totally intended). Kreisberg makes another mention of that fact earlier in the interview:
DEADLINE: So, I have to ask, with Batman V Superman having opened big on the big screen and WBTV’s Gotham being over on Fox, is there a Batman in Flash and Arrow’s universe?
KREISBERG: (Laughs) The answer to that is above my pay grade.
Which is completely weird considering the topic of discussion amongst the scoobies immediately went to "How do I jump to a different Earth", which is exactly what Barry just did by accident. This super smart CSI doesn't realize they could just figure out how to control it, like they did with time travel. Instead, they have to give Cisco more lines than necessary.
Update: Given that the CW has taken over for Super girl season 2 and are planning a multi show crossover, this will be fixed soon. See my answer at Biggest single episode TV crossover
[A] giant four-show mega crossover event is headed our way that will bring together Supergirl, The Flash, Arrow and DC's Legends of Tomorrow.
On a conference call with the press this morning, CW head Mark Pedowitz confirmed that all four of their Greg Berlanti-produced DC series will join up for one big story. Said Pedowitz, "Just imagine what our superhero crossover episode is going to look like come December," while also referring to the crossover as "the biggest one ever."