For me there are two basic reasons: 1) everyone I know is on iMessage, 2) iMessage does everything I require of a messaging app.
Honest question and not trying to be hostile, why should I prefer telegram? What am I missing?
So you're privileged in an entirely privileged bubble? If iPhone is only 52% of the USA and 20% of the world it seems almost statistically impossible that all of your friends or family can use iMessage.
https://www.reuters.com/article/us-apple-fbi-icloud-exclusiv...
https://support.apple.com/en-us/HT202303
"Messages in iCloud also uses end-to-end encryption. If you have iCloud Backup turned on, your backup includes a copy of the key protecting your Messages. This ensures you can recover your Messages if you lose access to iCloud Keychain and your trusted devices. When you turn off iCloud Backup, a new key is generated on your device to protect future messages and isn't stored by Apple."
Although Apple uses end-to-end encryption for both iMessage and FaceTime, it doesn’t do the same for iCloud backups. They are encrypted, but Apple holds the key, meaning that the company has access to a copy of almost everything on your phone – and that includes stored messages.”
The situation of not knowing a single non-Apple user is unlikely outside the US anyhow.