Because I do not want my governments, at any level, which all represent the ENTIRE community that elected them, to pick and choose which belief systems they will display (and thus at least appear to support), and which they will not.
City Hall, or the Legislature, or the Court House, or the state run schoolroom, must not selectively give appearance of support for one belief system, when their community consists of all belief systems entertained by all their members.
Right now, in a public square in Toronto (Dundas Square) there are Christmas decorations along with a Menorah. The Menorah is there, of course, but just now we're also celebrating Hanukah. I happen to approve of that. There will soon be a Nativity scene as well, and that's just fine with me, too.
Where I went to school, a boys private boarding school run by Quakers, we had Christians (Catholics, Anglican, Protestant and others), Jews, Muslims and "others." My roommate was Jewish, and among my favourite people at the time. Our "holiday celebrations" mentioned both Christianity and Judaism, but were primarily secular and all-inclusive, which is still how I like things.
Please understand me. I do not wish to poop on anybody's party, and seeing others really enjoying the traditions of their various faiths makes me quite happy. I even love to participate, as best as I, an atheist, am able.
But governments, like my school, must scrupulously represent all of it constituents, without favour and without discrimination.