Think of it like a big public message forum. Technically it works like one: You post messages and they're stored on the server and served up to everyone else at any point in the future. It's perfect for coordinating big groups of strangers.

It has a good reputation for refusing to kick people off when governments demand it. Although accounts are tied to phone numbers it doesn't show them by default and users can choose a unique username, so unlike WhatsApp or Signal it's easy to recognize the pseudo anonymous identity of people you don't know. It's much easier for many people to use than Twitter.

When running an opposition movement it doesn't matter if there's no encryption. Since you are open to the public your adversaries are going to be in your groups anyway. Another bonus is that most of your followers aren't already using it, which makes them more free to act. If a different messenger is popular in your country you might not want to tie the long held account that everyone in real life knows you as to your freedom fighting activity.

There isn't necessarily am answer at all. As these people aren't cryotonerds they likely just use the first one their associates use. Telegram was one of the earliest e2e encrypted chats to gain some notoriety as a means to avoid surveillance, and the publicity from that and countries banning it provide advertising others lack.