they have higher drag coefficient (i.e. they're less hydrodynamic) because of their, ahem, "larger members".
either that, or they lack the european gene that makes males think it's OK to parade around in a speedo.
aside: ladies, what you you think of men in speedos? still a no-no, or have we gotten over that knee-jerk reaction? has the euro-trash look become popular enough for the stigma of the speedo to be removed from the mainstream?