As pink grapefruit was only discovered/commercialised in 1930 perhaps this drink was made originally with yellow grapefruit? Or maybe it's an example of 'the latest trendy fruit' cocktail of its day? Combined with the popularity of the 'grapefruit diet' of the time (a sort of starvation diet that would not be recommended today, but endorsed by Hollywood celebrities and featuring in various period films) perhaps this explains an uptick in cocktails with grapefruit around this period.