The same reason IPv4, Unix, and unencrypted email are still dominating: it's good enough, and it's available everywhere, with no fuss.
Coding over SSH is just so convenient. It doesn't matter whether you're using a Raspberry Pi or a DigitalOcean VM.
When you're coding in vim, you're not losing all of your IDE and just getting vim. You're losing all of your IDE and getting all of the Unix userspace. Unix is a great IDE.