You can click on the PEER1 Hosting logo*:
(*It's clickable on the actual Stack Overflow page footer, not on the above image.)
Which gets you to this page:
The circled link on said page leads you to a blog post written by a member of the SE team that explains it:
When we decided to move our Stack Exchange Network to the East Coast
to better serve our global customers, PEER 1 Hosting was the logical
choice because of the success that Fog Creek had. We began to migrate
part of the data center in May of 2010, and finalized the move of all
live sites from Oregon in October of 2010. After all the sites were
set up at PEER 1 Hosting, we noticed some awesome results and thus we
started a discussion with PEER 1 Hosting about how to extend the same
benefits to the community.
We think it’s a win-win!
As an advantage of being part of the community you get an awesome data
center at a discounted price – Win!
The more business Peer 1 Hosting does with people in the community, the
more support they can provide to power Stack Exchange – Win!
Basically, they found PEER1 to be a fantastic enough service to advertise it (for free?) via the footers on Stack Overflow and Server Fault. It doesn't show up on other sites probably because they are not of the demographic who would buy PEER1 hosting services (i.e. Cooking.SE, Home Improvement.SE, Bicycles.SE, etc).