Because that's the name he chose when Freya asked who he was:
Doctor: I'm looking for Grendel.
Freya: Only a warrior would seek that which others flee. What is your name?
Doctor: I'm glad you asked. My name is Schweitzer. Doctor Schweitzer.
Freya: Schweitzer.. A hero's name. Come, I will take you to the kind.
The name comes from Dr. Albert Schweitzer, and was chosen by the writers because it sounded funny:
One of these jokes was that the name The Doctor assumes in this episode, Schweitzer, is a reference to Dr. Albert Schweitzer, a medical doctor, philosopher and recipient of the 1952 Nobel Peace Prize. Naren Shankar recalled, "I was going through the names of famous doctors in history, and Albert Schweitzer sounded really funny. I thought it would be cut, because The Next Generation was never really known for its humor [....] Here I was, writing these scenes with the Vikings all shouting, 'Schweitzer, Schweitzer!' and thinking, 'They're going to cut this; there's no way this is going to make it,' but it did." (The Official Star Trek: Voyager Magazine, issue #4, p. 67)