Alternatively, they don't understand they need to make themselves accessible and just assume when they get no interest from learning disabled people, that there aren't any learning disabled people interested.
In reality, they may not have advertised in a way that would be accessible. For example, they may have used complex language or not promoted the project in places learning disabled people have access to. Or they may have made the project seem like it would be inaccessible, difficult or unappealing to learning disabled people.
This is very unfortunate, because we are literally losing history.
Yes, learning disabled people have access needs, but that doesn't mean their stories shouldn't be remembered.
It may be that they need things communicated in a more accessible way. Maybe they need things explained differently or broken down more. Maybe it will take them longer to process and think of answers. It may be that they need to have a couple of shorter opportunities to share their experiences, rather than one longer interview.
Some ways to manage this would be to communicate with learning disabled people, those who support them and learning disability organisations, to make things accessible.