Titans' premature end sadly wasn't too much of a surprise. It's likely that it had nothing to do with ratings, popularity, or any other thing that goes into deciding a show's fate. The truth is that there are major changes going on at DC right now, and it all has to do with last year's Warner Bros./Discovery merger.
No official reason as to why Titans season 5 isn't happening was ever given, but the cancelation happened after major HBO Max shake-ups. Titans was originally part of the DC Universe streaming service, but it moved to HBO Max after DC's streaming platform was discontinued.