First and foremost, please don't think I hate Superman or DC, because I don't. Secondly, I need to elaborate on this case so stick it out until the end and we'll discuss it in the comments later.
Superman is the central figure in the DCEU
Yes, you heard it right (this point might not make sense initially but you will understand when you connect this with other topics that I am going to discuss below). It might not be Batman, because Superman's arrival and actions on Earth during Man of Steel triggered a strong chain reaction that resulted in Batman v Superman: Dawn of Justice. It's because of Superman the entirety of the DC Extended Universe is coming together—in other terms he is the catalyst of this cinematic universe.
Batman v Superman: Dawn of Justice is about Superman!
Sure, Batman has slightly more screen time than Superman in the movie, but when you think about it and when you understand the messages behind the trailers and other promotional materials, you will realize that this movie is about Superman. It's about his place on this planet, it's about how humanity is split in two because of his arrival and actions during the events of Man of Steel, it's about how governments are viewing him as an illegal immigrant and potential danger, it's about how he ultimately becomes a globally accepted hero at the end of the day.
Thus the crux of the story is centered around Superman and why Batman and Lex Luthor are going up against him. This would have made Zack Snyder say: "In a way Batman v Superman is Man of Steel 2"
Most of the Justice League villains are Superman Villains
There are some exceptions to the case, but when you see the Justice League storylines - most of the time - you hear about Darkseid, Brainiac, Mongul, Doomsday, Imperiex, Lex Luthor and their agenda of taking over the world and destroying the Kryptonian in particular.
Due to this reason, Superman ends up serving as the backbone for many Justice League storylines. It's heavily rumored that Brainiac and Darkseid are lined up to play the big baddies in Justice League 1 and Justice League 2, thus we can expect the whole of the Justice League to be Superman centric in some capacity.
Solo movies on Superman are complex and tough to crack
At his core he's an indestructible being of immense power and to match his strength, most of the threats he faces are intergalactic or Justice League level in nature. Having such a great threat with so much at stake sounds like a solo outing for a superhero, especially in a shared universe. Didn't get me? Okay. Imagine the events of Man of Steel taking place after a solo movie. Everyone would be scratching their heads over why Batman and Wonder Woman didn't show up to help Superman, right (plot hole)? That's the point. His villains are so monstrous and out of the world that it essentially becomes a Justice League level threat, thusly, story.
Secondly there's this general idea to keep every superhero out of a solo outing other than the title character to show how this hero is cool and handles stuff on his own and also to keep the entire focus on the title character itself.
Lastly, watering down the abilities of Superman to face lesser threats like Parasite, Toy Man, Inter Gang, Conduit or others in a solo outing might look inconsistent—and on the other hand many of those above mentioned villains are not relatable at all, not to mention campy.
WB's focus is on expanding the franchise through different movies
When you a look at the slate above, it looks like WB intends to build this universe through different characters and stories, by giving movies to all Justice League members, but not sequels to their solo outings. Recently we heard that WB is aggressively looking towards producing movies on the Dark Universe (Justice League Dark) and Batman, focusing on pushing these two movies for the current slate.
Don't think that WB doesn't like Superman, or that they are neglecting him. They're not. He had a solo movie and will have a major part to play in at least three more movies. In fact, he's the catalyst of the DCEU—it's just that putting him a solo movie in a shared universe is tough and complex do to the above mentioned reasons throughout this article.