They were both Americans from the 20th century, they were both supporters of peace and social justice, and they were both orthodox Catholics. Although Merton isn't particularly known for it, he cared deeply for the poor just like Day. He also made great ecumenical efforts with eastern religions which ties into Pope Francis' message of outreach. That's probably why they were brought up (although admittedly I have yet to watch the speech). I know much more about Merton than Day since I've read his books on my own and for school.
By the way, if you're at all interested in Christian spirituality I highly recommend reading
No Man Is an Island and/or
New Seeds of Contemplation. Of course there's also
The Seven Story Mountain which he is most known for.
You can find a lot of Dorothy Day's writings here:
http://www.catholicworker.org/dorothyday/browse/index.html
