The reason for this is mainly contextual. Deadpool was purpose-built to do, say and behave in the outrageous way he does. That same "hook" was conversely grafted onto this particular animated iteration of the Titans, so it's fair to say that it feels less organic to fans, particularly those whose first impression of the characters came from the comic books or the previous animated series.
Teen Titans, which ran between 2003-6 on Cartoon Network, earned a particularly warm place in fans' hearts and -- despite having a respectable five season run and a spinoff movie -- is sorely missed to this day. This adoration isn't misplaced, either. Teen Titans had strong writing, emotionally-involving character arcs and a unique, anime-influenced visual style crafted by showrunner Glen Murakami (dubbed "Murakanime" by fans). The show was also incredibly funny, too, and that same vein of oddball, anarchic humor was pumped directly into TTG!.
Not only do some fans still resent the lighter, wackier version of the Titans replacing the more serious one they fell in love with, they also hold a grudge against it for replacing another fan-favorite DC cartoon: Young Justice, the cancellation of which came in the same year TTG! began airing. While Young Justice's maturity was a hit with older viewers, it reportedly didn't connect with the younger, toy-buying demographic the network wanted. (And, if you believe Paul Dini, there may have been some gender bias at the executive level going on.) Although Young Justice's axing was gutting, and its planned resurrection more than welcome, the fact that a TTG! spinoff movie is getting a wide theatrical release this year shows it's working as well as Cartoon Network hoped it would, if not better.
Just like Deadpool, which -- after being stuck in development for over a decade -- hit theaters just when superhero fatigue was showing early signs of setting in, the TTG! movie is coming hot on the heels of a string of DC live-action movies that turned a certain proportion of the audience off. The overblown seriousness that some fans critiqued the DCEU for was expertly parodied back in the 2015 TTG! Season 2 episode "Let's Get Serious." The story saw the Titans adopt scrunched up, unsmiling faces, beefed up bodies and wallow in their conflicted psyches while stalking criminals at night... despite not being able to see very well because, well, it's dark at night. They even had a crossover encounter with some Young Justice members, resulting in a great meta-moment as Cyborg and Aqualad voice actor Khary Payton had a conversation with himself.
That level of self-aware buffonary is exactly what made people fall in love with Deadpool in 2016. The only difference, really, is that Deadpool's R-rating seems to give its gross and hyperactive antics the free pass that has never been given to TTG! by a viewership that isn't the cartoon's target audience anyway. Like it or not, TTG! is, for the very youngest fans, an accessible and entertaining gateway into the DCEU, and, for the seasoned ones willing to give it a chance, a breath of
fresh farty air into an Extended Universe that could use some lightening up.
Debuting in theaters on July 27, 2018, Teen Titans Go! to the Movies is directed by Aaron Horvath and Peter Rida Michail and stars Will Arnett, Kristen Bell, Greg Cipes, Scott Menville, Khary Payton, Tara Strong and Hynden Walch.