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.
Apple doesn't have access to your iMessages unless you give them permission by consenting to have your keys backed up to iCloud. If you use iTunes backup (or don't back up your phone), the Messages will be encrypted and can only be unlocked if you have a device that still has the keys.
"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."
“There’s always been one major problem with Apple’s privacy claim that ‘What happens on your iPhone, stays on your iPhone‘: it isn’t true of iCloud backups.
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.”
I don't know what "gimmicks" it may have which iMessage does not(although the in-chat polls are pretty handy), but IMO the big strength of Telegram is privacy. You can use it without revealing your e-mail address or phone number.
The situation of not knowing a single non-Apple user is unlikely outside the US anyhow.
Why don't Americans use Telegram?