Sadly, because it makes a ton of sense. It's the same way Apple got so many to use iMessage. People on iOS generally doesn't realize that iMessage isn't SMS.
I really like Signal, but it's a niche thing for me. I have two friends who use it, familie is all on iMessage or SMS and SnapChat, that's it. Signal would make sense for those on SMS still, but convincing them to switch is going to be an uphill battle. Those on iMessage, why would they consider using Signal, when everything just works.
In my family circle at least, WhatsApp outweighs SMS and iMessage by at least 2 orders of magnitude. In the Chinese-speaking world, Tencent/QQ is the messaging/payment/commerce app that subsumes everything else, with iOS and Android being like the invisible and mostly irrelevant BIOS layer underneath.
I think when parent wrote "People on iOS generally doesn't realize that iMessage isn't SMS" they are not talking about so-called technically-literate people.
The very fact that you're commenting on HN probably means you're technically-literate so probably it does not apply to you.
But I am also curious about that sentiment about not realizing iMessage isn't SMS that. I heard it repeated a lot. I have not met a lot of people that share this sentiment and have not heard of research around this specific topic. So I'd love to put some data behind it.
One does not have to be tech-literate to notice the limitations of green bubble SMS/MMS versus iMessage, particularly in group chats or when you want to send or receive photos (often recompressed and received in a lower resolution on MMS), videos (very low file size limits on MMS), or any arbitrary file type (often not possible on SMS).
Data would be interesting, but we'd also need to know what people think SMS is. For my mom, SMS is the text messaging app on her iPhone. My sister knows that it's iMessage, but she doesn't care, it's still the app that allows her to send a text message, it might be iMessage, it might be SMS, but practically speaking, it's a text message. My guess is that it could be IRC or ICQ for all she cares, it move a message from her to a friend or family member. The important part is that it's able to reach anyone with a phone.
I really like Signal, but it's a niche thing for me. I have two friends who use it, familie is all on iMessage or SMS and SnapChat, that's it. Signal would make sense for those on SMS still, but convincing them to switch is going to be an uphill battle. Those on iMessage, why would they consider using Signal, when everything just works.