Why often is used a varistor 7D471K for 220-240vac surge protection rather than a lower nominal voltage one?
You must be reading the specs wrong. A 10D221K would blow up when connected to 240VAC mains as mains have about 340V peak voltage, and so something else must be used.

10D221K can handle max 140 V RMS or 180V DC, and starts conducting 1mA at voltages between 198 and 242 V. It will clamp to 360 V, when 25 A is flowing. It just is not compatible with 240 VAC mains.

7D471K can candle max 300V RMS or 385 VDC, starts conducting 1 mA between 423 and 517 V, can clamp to 775 V when 10 A is flowing. No problem at 240 VAC mains.