SMS (in it's classical form) is highly spoofable as has been proven with TAN codes time-and-time again. It will work most of the time (sms has no guarantee of delivery as far as I know) and is somewhat secure but by no means good in any way.

This relies on email, which is highly secure, smtp is not spoof-able, server and mail origins are traceable and verifiable and there is no way to intercept the message on a malicious router/switch whatever. Ow wait ...