There is a winter holiday tradition of kissing under the stuff. So
why it is never associated with a holiday devoted to romance? The
supply cannot be an issue. It is still green and healthy, and it
would be just as easy to harvest it now.
The same thing could be said of, for example, holly leaves and berries
as decorations. Why do the winter holiday traditions and decoration
end at the new year? It is the same weather and environment. Holly
and mistletoe would be far superior as decorations than everything
else you see associated with this holiday.