It can be said comfortably that a majority agree that the Marvel Cinematic Universe (MCU) makes better films than the DC Extended Universe (DCEU). Why is this though?
There are many arguments, but I will focus on one aspect-the tone of the films. The tone of a film is extremely important in gaining the attention of the audience. One of the major highlights of the MCU is levity in the tone. The MCU has demonstrated great strength in blending humour along with action, and the end result is a visually satisfying experience. The tone of the entire MCU was all set with the release of their first film, 'Iron Man' (2008). Nothing about the tone of the MCU has changed in 11 years, even with the recent release of 'Avengers: Endgame' (2019).
The DCEU on the other hand, had its first two films, 'Man of Steel' (2013) and 'Batman v Superman: Dawn of Justice' (2016), begin with serious tones. 'Suicide Squad' (2016) was released with a rather comedic tone, with a pinch of seriousness. 'Wonder Woman' (2017) was released to a rather grandeur tone, and was also noticably 'lighter' in tone. 'Justice League' (2017) could not make up its mind with tone. It did a bad job of mixing seriousness and comedy together, which was one of the reasons for the film's failure. It was not well-received by viewers.
The problem with Justice League is that it messes up character arcs from previous films. Superman from 'Man of Steel' was one that was discovering himself, confused and scared of himself. He thought of himself as an outcast, that he would get rejected by others around him. He had a tough childhood. He was different; he was a literal super-man, a literal super-human. In 'Batman v Superman: Dawn of Justice', he actually was rejected by the entire world. All that could be present within him was darkness, sadness and fear. Batman in the same film was one that had regret, sorrow (due to the loss of Robin potentially, as seen in the film) and wanted revenge. He was full of vengeance. In 'Justice League' however, both of their character feelings and emotions changed, particularly Batman. They both all of a sudden became comic relief characters. Many were enraged at the portrayal of Batman in particular.
There may be another reason why this sudden change in tone of the DCEU happened, and that reason is the MCU itself. We must keep in mind that the MCU has been released before the very existence of the DCEU, and their movies have also been receiving acclaim from then itself. The films were praised for their light-hearted tones, and that praise continues till date. Due to number of MCU films receiving praise for the same reason, the DCEU might have had to change its tone as well. Not to mention that between when the DCEU released 'Man of Steel' and 'Batman v Superman: Dawn of Justice', the MCU had already released 5 films.
I am not saying that the reason for the DCEU's fall is its seriousness. The Dark Knight trilogy had the same tone, but all three films were highly acclaimed. It may have been a matter of just bad timing. The MCU had already established themselves as the leaders of the superhero genre. Casuals pretty much expect humour in superhero films nowadays. It really is only the hardcore comic book fan that does not mind the lack of humour. DC Comics already have a reputation for its darkness, which is something fans love.
The DCEU may have had no choice but to conform with the 'superhero norm', and give what the general audience wants. This change is observed with the films 'Aquaman' (2018) and 'Shazam' (2019), which have light and fun tones.
Another problem now is that the hardcore DC Comics fans want serious films, so that they are faithful to their original source material. In terms of actual viewership of superhero films in theatres, the general audience is higher in population, so filmmakers are going to listen to the dominant group. There is pressure on the filmmakers of the DCEU to 'be like the MCU'. When everyone talks about superhero films in general, or when the term 'superhero' itself is brought up, the MCU is what pops up in many people's minds. The MCU has established itself as the biggest in the world of the superhero genre, and dethroning it will require a lot of time and effort.
Now we have seen how tone alone can change the course of a major film universe. Excluding tone however, there are other problems with the DCEU and why it has a tarnished reputation, which is an article for another day.