That last reason may be because shirozake is very nourishing. Like the nerizake from ancient times, it can almost be eaten rather than drunk!
Shirozake is strictly speaking not "nihonshu," as it is made from a mixture of steamed glutinous rice, mirin, koji and shochu (distilled liquor). After maturing for a month, this mixture is then crushed in a mortar. It contains only 10% alcohol (but is considered as a type of liquor) and almost half of the pulpy mixture is a sweet rice porridge.
I am not so fond of porridge and in order to get into the proper seasonal "Hina" mood, I prefer to have a glass of daiginjo fruity sake with a petal of a plum blossom floating in it...
P.S. The largest and most famous provider of Shirozake in the Edo-period was Toshimaya, founded in 1596 and then located along the Kanda riverbank. The company still exists and has branched out into soy sauce and frozen foods. Although the brewery has moved to the suburbs (Higashi-Murayama City), it provides the Imperial House every year with Shirozake and also brews the Kinkon Masamune sake that is used at the Kanda, Hie and Meiji Shrines.