Why isn't Signal bigger? (currently living in a country that monitors everything and looking for some relief)

Network effect. There's more reason to sign up for WA/FB when everyone you know also has an account.

Why not email?

If signal users can be counted on one hand, people using gpg can be counted on a finger.

I don't see a need for GPG at all. There are tons of options for sending email that is encrypted enough to lock out the Iranian government. Any popular mail service would work, and using several email addresses at different mail services would work best.

Also, organising a protest isn't about keeping individual messages secret. You can never keep something secret that you want thousands or hundereds of thousands of people to know about. It's all about getting the message across fast without the government blocking it.

And without knowing who you are, a requirement most services (including Signal) fail. Does Telegram allow you to create an account without divulging your phone number?

I don't know. I'm not a Telegram user and I wouldn't use any of these messaging services if I wanted to organise a protest.

Email is the only reasonably decentralised and diverse messaging infrastructure we have. It's impossible to block without shutting down the entire Internet in a country.