They also just don't have the rights. I would think that time will eventually run out on Marvel Superhero Island because of the MCU. I know they're different entities, but most people don't get that, at least I think. Confusion may become a problem, but it also houses 2 of their best attractions so who knows. My ultimate dream is by 2025/2026 DC gives their theme park rights to Universal, and we get a full on Gotham City.
I personally am not loving how Disney has handled the MCU in theme parks right now, so I personally and quite hopeful that DC can be represented fully one day. Or, just wish Universal had all control over Marvel. We'd have an "Avengers Campus" by now id feel.