it's analogous to how some lgbtq or black people join the US Republican party. people don't always see their own core identity as what others perceive as their most core identity.
for instance, because nowadays there is less of certain types of institutional oppression constantly telling black men "you're black and that's terrible", their self perception may instead center on being straight male (not to say that they forget about their racialized identity or don't care) and they might focus and worry more about maintaining patriarchy than freeing black people.
similarly LSN NDs may identify more with neurotypical out of an instinct (often subconscious, but sometimes conscious and malicious) to preserve their already threatened social status within ableist society, rather than to join HSN NDs in the upcoming long arduous fight to raise up all NDs.
this fight has barely begun and even though society less frequently commits blatant atrocities against NDs than in the 1800s/1900s, many horrors are still to come.
edit: clarification