Because Cartoon Network wanted to get rid of it.
The show didn’t flop; since the beginning, it’s ALWAYS had an active fanbase stemming from the original 2013 pilot. Once the show premiered, I remember constantly seeing GIFs and fanart, so I know the show wasn’t a failure.
CN told the OK K.O.! Krew their show was cancelled barely A YEAR after it finally premiered. The fact that they aired 70 fucking episodes (https://twitter.com/ianjq/status/1024817887020642305
) during that first year means CN had a huge backlog of finished episodes to air, so they likely could've premiered the show as early as 2016. That would've given the show just a little more time to build an even bigger following, but nope! CN decided after ONE YEAR that this show was “dead weight” and cancelled it so they could make room for shit like Victor and Valentino, Tig n' Seek, and The Fungies!...
The network sabotaged what could’ve been the perfect mix of Teen Titans Go!’s wacky superhero comedy and Steven Universe’s down-to-earth stories with occasional emotional depth, but for whatever reason, CN didn’t feel the need to support it and instead chose to focus on picking up tons of new, babyfied cartoons.