[deleted] Can anyone explain me why has the most (recommended) secure mobile OS (or let's call it a custom ROM+) hasn't received a much wider adoption by now?
Because to have GrapheneOs user have to do something extra in comparison to use default. As an example we can take Google's multibillion deal with Apple to just have Google set as a default search engine.
The other factor is, I believe, that technology in this case mobile OS is widely used, yet the rules and implications are obfuscated, hidden or not as easy to realize in comparison to simply use the technology.
What I mean by that is that a person can be given a phone and even though the person never had a phone before, s/he can use it successfully. So s/he can send e-mails, call, text, play games, do banking and everything that you can do with a phone, yet that person can be completely unaware that the little thing in its hand (no joke intended) can collect and share incredible amount of personal data. The two main company in the field Google and Apple do so little or even nothing to stress this important point. Google in this case is more transparent, but do not use it to its advantage. Apple is a marketing company, so they usually do not comment or simply lies about its practices.
It is really easy, simple and comfortable to stay with defaults and not to care. Once you start caring, you do something.