Because your camera's meter is trying to push a very bright scene toward a middle tone, so it underexposes and the snow comes out grey. The fix is exposure compensation: dial in somewhere around +0.3 to +1.0 stop as a starting point and check the result. There's no universal number, metering systems vary, and some scenes need more while others need none.
Use the histogram and the highlight warning rather than the rear screen, which misleads badly in bright sun. In a snow scene the histogram sitting well to the right is normal and correct. What you're avoiding is pushing important highlights so far right that the texture disappears. Ice should read bright and still show detail, not become a blank white shape.
When dark wildlife fills a lot of the frame the exposure shifts again, so review as you go rather than setting one value and trusting it all day.
If you learn one camera control before this trip, make it exposure compensation. It does more for Antarctic photographs than any lens.