2FA is not very helpful if your service relies on encryption for everything (like keybase). It's not possible to use 2FA without another party verifying the authentication code. Keybase could include this (then they would have to verify it), but so far keybase has gone to great lengths to not actually have to hold any secrets for users. They even perform password hashing in the browser instead of on their servers. 2FA would force them to do some processing with secrets on the server side.

Having 2FA with Keybase would be similar to having 2FA with a local password safe like Keepass. Yes, you could implement 2FA. However, you would have to rely on an external service, and if people have your password and can modify the software, they can simply comment out the 2FA check and they can decrypt your password safe.

The entire security model of Keybase is based on having all data encrypted at all times. Adding 2FA would add some false sense of security, which doesn't affect the encrypted data at all.