The narrator does not claim to know Sven's motive — “Don't ask me why Sven wanted a pet, or why he chose the one he did. I'm not a psychologist.” What he does say is that the choice was “very sensible”: a canary “weighed practically nothing, her food requirements were tiny — and she was not worried, as most animals would have been, by the absence of gravity.” On a station where everything is rationed by mass, that made her the one pet that could be hidden and kept.
Sven's own stated reason was scientific curiosity. He had smuggled her up on the last ferry from Earth after his leave, “partly, he claimed, out of sheer scientific curiosity. He wanted to see just how a bird would operate when it had no weight but could still use its wings.”