Ukraine's National Coordination Center for Cyber Security on September 20 banned the usage of the Telegram messenger app for state officials, military personnel, and employees of key infrastructure, citing security issues.
Sure, and I use Tuta. Those are outliers, the vast majority use gmail, or at least the vast majority in my circles do.
It's the same thing as the network effect, just a little less ubiquitous, people will tend to use whatever everyone else uses. Getting something new like email (SMTP) is a huge endeavor, it's a lot easier to just build a centralized service and get people to use that, and most people will use the same provider anyway.
I don't like it, but I understand why it works and is so common.
No, those being outliers means the email argument isn't particularly strong, especially when talking about a new standard. If most people use a single service anyway, why would a company go out of its way to make something decentralized? And for something like encrypted chat, that's a lot of extra work.
What, by starting as a government system using a completely different protocol, then adapting to always-online network connections (i.e. universities) at a time when spam didn't really exist?
The 70s and 80s were a very different time, and regular consumers didn't use email until it had gone through several iterations. Even so, most people used a single implementation (sendmail on BSD) for quite some time before anyone else got involved.
The internet today is a very different beast, you can either try for an open standard, or you can try for user acquisition. Almost nobody seriously goes for the open standard anymore, unless it's an iteration of an already existing open standard.