Because it isn't an universal celebration , such as Cinuqo de Mayo(spelling ?). For example in my country mother's Tag was on the 8th of march. So I guess Auszeichnungen go just for universal celebrations , as Weihnachten , Easter , Valentine's etc.
I assume it's because Mother's Tag is on different days for different countries. I only found out this today though, because of everyone on here saying 'Happy Mother's Day!', but here in England our Mother's Tag was in March, so I guess that they wouldn't be able to choose one specific day, it would just be confusing to those who had Mother's Tag on a different day.