I would say the same reason there is no World of Warcraft 2. Think of the Episodes as sequels to the original game. The only way for a possible sequel to this game is when DCUO loses subs and have to shut down their servers.
But there is Everquest 2 and the original Everquest is still going strong. So DCUO 2 could happen, while this one still going. People wouldn't just give up their progress, but they will invest into DCUO 2. But I think they need to focus on this DCUO, fix what needs to be fix and add more features and stuff to do between Episodes & Seasonals.
I'll add to that : why would Warner give Digital Ink / Daybreak another DC license when, contrary to more than 10 years ago when DCUO was still under development, they now have their own first party studios with a lot more funds and workforce than Digital Ink has ever had ?
This game is old enough, obsolete enough and struggling on p2p enough that a real sequel could happen. Particularly MMO and console gaming advanced tremendously since the launch. But they can build much better mechanics and classes without having to break the current game which would be nice. This game is the most basic of basic, current f2p and p2w content games are so far ahead of this one now in graphics, gameplay, content,etc
Incoming Novel, sorry Context is important. WoW does not have a sequel because it is an anomaly. WoW does not have sequel, because it is a triple A title which is still highly profitable. Final Fantasy 11 has also been out for quite some time, but has a sequel despite being way more populated and profitable than DCUO. It is also sub based. The game usually has nearly 1,000 people per server. 16 servers total, with a standard sub fee of about $12 to play. You do the math. Not every battle royale game hits the jackpot, which leads to the studio creating their own platform. Not every game hits the lottery like Fortnite. Just because Overwatch may release a sequel, does not mean Fortnite will, or any other battle royale game because their circumstances differ. Assuming every studio rakes in the same revenue, sure, DCUO is comparable to WoW. We all know that is not the case. Fornite has contest giving away enough money to fund a DCUO part 2. The reason DCUO may consider a sequel is due to the superhero mmo void. There is currently not a single successful superhero mmo on the market. Whoever fills that void will be swimming in money because there will be literally zero competition. I seen mmo come and go. Dragon Prophet is a game loved by the fans, but will be closing its doors this month. It is a Day Break title. Just because there are loyalist who stand and swear by DCUO, does not mean it is not warranted of a sequel, or slowly phasing out due to financial obligations. I do think the DC IP is too valuable to simply give up on without a backup plan like DCUO part 2. I doubt this game will shutdown unless there were a part 2 in the works. Even then, it depends on how profitable it is. A case for a DCUO part 2 is to keep the DC IP up to date with the modern tech. That is why FFXIV and Guild Wars sequels exist. Not everyone is the anomaly that is WoW. Other games simply don't have a sequel because the previous are not successful, or you have a WoW or TERA on your hands.
I guess you could always reach a point where all new content could be produced on a more modern game engine. Of course it would require re-rendering all current styles so everyone's characters would appear correctly. Also, it would be weird playing PS3/4 level graphics and then queuing up for something closer to cutting edge. Of course I don't know if any of this would be technically possible without a ton of money. A true sequel would probably perform better financially since grabbing new players would be so important.
Wouldn't mind if they give it to the studio that does Warframe. AAA level graphics, experience with action mmo, less pay2win, better community engagement.