Oh sorry, I forgot that Hanukkah, Kwanzaa, Winter Solstice, and Boxing Day were not real holidays.
What did I ever do to you? I'm not your enemy. And I'm just making observations. I'm not part of the whole 'war on Christianity' stuff.
Winter Solstice is a transitional spot on the calendar. It's the beginning of winter and the shortest day of the year. While there have been many traditions associated with it - including Christmas itself - it's not a holiday any more than the Summer Solstice is.
Boxing Day is only celebrated largely in a few British Commonwealth nations and its origins are - surprise - rooted in Christmas.
Unlike most holidays, which celebrate the anniversary of an event or religious observance - Kwanzaa was "created" in 1966 by Ron Karenga as an "alternative" to Christmas, because as he put it, Jesus was psychotic.
Christmas is the biggest holiday in the Western world and its strictly cultural and economic impact dwarfs everything else. Some nations and some businesses do almost all their commerce during this time. No other holiday has the same impact in the West. You could even eliminate everything relevant to Jesus Christ and Christmas would still be culturally and economically the biggest event all year long. Movies are released for the Christmas season, with Christmas themes. Sales and decorations adorn the whole country. The entire month of December is one long cultural festival.
Take it away and - what? - we're having a Passover party? we're gearing up for Ramadan? Take away Christmas and the whole season vanishes. It'd be like taking gambling out of Vegas. No one would show up.
Take it away and Kwanzaa, Hannukah and Boxing Day have less cultural and economic impact than All Saint's Day. This is not prejudice against the other holidays - they simply don't have the impact that Christmas does. If the whole world decided to move Christmas to say, April, the whole Happy Holidays thing would get a shrug from shoppers. Nobody would care.