Titans could have had a much more significant role in the Arrowverse's Crisis on Infinite Earths event, but it was a good thing that the Titans' characters' cameos were blink-or-miss moments instead of an actual crossover. While both Titans and the Arrowverse shows share a key executive producer, Greg Berlanti, they represent two different corners of DC TV. The Arrowverse shows all aired on the CW and feature a high level of interconnectivity, while Titans was produced as a streaming-only content series set in its own universe.
Before HBO Max, DC had launched its own streaming platform, the DC Universe. As such, the company needed new, exclusive DC content for the service – and thus, Titans was developed. The DC Universe streaming platform was eventually discontinued, but Titans remained as a DC flagship TV show now for HBO Max. Despite the fact that Titans was produced for a different platform and was set in a different universe, audiences never ruled out that the show could crossover with the Arrowverse somehow, especially considering how shows like Arrow, Flash, and Legends of Tomorrow were constantly dealing with alternate realities and the DC's multiverse.
Some characters from Titans eventually appeared in quick scenes during the Arrowverse's Crises on Infinite Earths event through archived footage, but recently Nightwing actor Brenton Thwaites revealed that he was invited to have an actual appearance in the CW crossover episodes - an invitation which the actor declined. No other actors from Titans shot anything else for Crisis, and the blink or miss cameos were all there was for the show's crossover with the Arrowverse. While that may sound disappointing for those who wished to see the Titans interacting with other DC heroes like Amell's Arrow and Gustin's Flash cutting whatever was planned for Titans in Crises on Infinite Earths was the right decision. Not only was the massive crossover event already full of cameos and storylines, many of which were getting in the way of the most important, separate stories of each Arrowverse show, but the world of Titans and its characters would not blend well with the themes and the more fantastical approach of the Arrowverse.
Titans was obviously not a stranger to fantastic elements, as the show dealt with superpowers, magic, and resurrections from the start. However, the overall Titans' aesthetic and the more serious approach to the characters and the world around them is very much different from the Arrowverse, especially after things like time travel and aliens became so common to the lives of Green Arrow, Flash, and all the other Arrowverse characters. On top of that, Titans was written in a much more different format than the Arrowverse's usual 23-episode seasons, with fewer episodes and almost no side-missions, meaning that finding a place in the Titans' timeline to explain a huge crossover with other DC heroes would be almost impossible. Most of the Arrowverse shows had interacted at some point, and most of the big cameos like Clark and Lois from Smallville were from already concluded DC stories.
Crises on Infinite Earths was a landmark for the Arrowverse and DC in general, but writing characters from Titans into the crossover event would have been a disservice to both the CW shows already involved in the story and Titans. Not every cameo in Crises on Infinite Earths had the same amount of screentime, something that would have been an even bigger problem with the addition of the Titans. Having Titans being confirmed as an official part of the Arrowverse's Multiverse was already a good-enough interaction between the shows.