This website is aimed at people in the United States (note the shipping form), for whom an acre is still the most common unit of land measurement. Most people in suburban/rural areas in the U.S. are likely to have a sense of what an acre is, while few would know what a square kilometer looks like.
Don't Americans run 100 m and 5 km and 10 km distances? I would have thought that would give most people an intuitive understanding of what a km is. Even the US military now uses kms.
