When it comes to superhero comics and accessibility, they do reach out to new readers to some extent but some of the biggest drawbacks to having such a vast, extensive shared universe (though this may not be true for all shared universes) is that it can be hard to track down a character. Especially if it’s one’s favourite character, speaking from experience when it comes to finding good comics about Tigra. While other story franchises have their own problems and drawbacks, things like DC and Marvel have accidentally made it harder for somebody to find what they want to see.
Not just due to racism and sexism but also because it can be really hard to track down one’s favourite character if they don’t have a long-lasting self-contained magazine series of their own and that they can be seen in multiple magazines. While this may not be unique to Marvel and DC as this can be found in other shared universes to a similar or some extent, that’s the pitfall of having an extensive shared universe. If a character appears in multiple books or magazines, it can be really hard finding them at all as I know from experience especially if you want to find good stories about them.
Self-contained stories and universes might not be much better either when it comes to finding a character one is interested in, but if they stay in one storytelling universe and just one storytelling universe that makes it easier to find them. Much easier if they appear in a certain story arc. Not much better, but still better than appearing in an extensive shared universe the way you do with Marvel and DC. Maybe that might be one of the reasons why Image, Dark Horse and other publishers abandoned the shared universe model as it makes it harder to find one’s favourite character.
Maybe not necessarily the primary reason, but that self-contained universes and stories are significantly easier to get into than if it were an extensive, shared universe even if that’s not true for all shared universes. But the problem remains if it were an extensive shared universe.