The unsubscribe option is only shown for senders with a high reputation:
This only works for some senders right now. We’re actively encouraging
senders to support auto-unsubscribe — we think 100% should. We won’t
provide the unsubscribe option on messages from spammers: we can’t
trust that they’ll actually unsubscribe you, and they might even send
you more spam. So you’ll only see the unsubscribe option for senders
that we’re pretty sure are not spammers and will actually honor your
unsubscribe request. We’re being pretty conservative about which
senders to trust in the beginning; over time, we hope to offer the
ability to unsubscribe from more email.
Also note that the preferred list-unsubscribe method is mailto, not http. Outlook.com for example only honors mailto.