Sometimes it is a cultural taboo, sometimes people just don't think it's important to list it in obituaries or other public announcements with everything else on their mind, sometimes the cause of death is not known, and sometimes it's complicated.
Even when something is listed on the death certificate, cause of death isn't always neat. I have loved ones who have one cause is listed, but they also had multiple other issues. The 'cause' would really be more like a death of a thousand cuts - it was one thing after another after another that piled up into something that is recorded like heart failure, but everyone close knows it was a lot more than that. Whether we discuss that with others depends on the person or our mood. It can feel dishonest and a wrong against the person who died to give the listed cause of death when we know there was a lot more involved. It sometimes feels better to not list something that we can't fully find the words to explain.
I've also had people I know who died young and I've never really felt a need for further details. Maybe it's cultural, maybe it's because I dealt with a lot of illness and deaths fairly young and thought about it a lot as a child in a way that there were always things I wouldn't understand & that's normal and okay. As a teenager, I lost others my age to black ice, pregnancy-related complications, SDS, and there was a couple that I never learned directly, though I heard different ideas from others based on what was going on in their lives. I never felt the latter did much good as all it seemed to do was fuel chatter and gossip rather than anything that might help prevent another one.
While I can see passing curiosity, that doesn't give anyone more rights to information. I think in many places there should be more talking about death generally, there are benefits to doing so, but to an individual grieving person, that's a personal choice if they want to do so and a desire to make a public announcement shouldn't come with any obligations to give more information than they want to give.