I think a lot of "normies" looked at the RT score, compared it to the other movies in the series, decided it wasn't something they wanted to see, and then passed on it.
It's not like the movie exists in a context-free zone. There are three other movies that are divisive at best before this, and one clear outlier that's actually pretty decent.
If WB/DC didn't insist on shitting the bed 4 times out of 5 people would give them the benefit of the doubt.
All they had to do was stich three decent-ish CW episodes together for each movie and they wouldn't be here. I'm actually kind of impressed at how badly they managed to fuck this all up given how good some of the source material and TV stuff (animated, mostly- the CW shows aren't great but usually manage to be entertaining at least) has been. But apparently they were having silo wars and trying to be different to the TV side of things because, you know, God forbid you do something that's already known to work.