Does anyone know if there's a reason why there's no MegaBonus in the month of May?

Spring MegaBonus goes until the end of April and Summer MegaBonus starts in June, meaning that stays in May don't count for any MegaBonus. For some reason, I tend to have a lot of stays in May, so I'd be curious to know if anyone knows why May is not covered by any MegaBonus promotions.