The reason is they r more active. Who wants to trade a pair if they never move, or move very little steps?
For the 2 gbp pairs, if u have caught the trend, even intraday, 30-60pips is average, can go over 100+, and u can be more confident to sell rallies and buy dips because reversal during a day is not that often.
You may also consider eur/jpy with smaller spreads but similar movements as gbp/jpy.