I'm a firm believer in holiday decorations coming down either on or just after New Years Day. Why do many keep them up longer into January?

Because some keep celebrating or are lazy. 

Christmas isn’t over until Epiphany (6th January).    I don’t like to take them down before then, it is unlucky.  In fact, it used to be the custom to keep them up until Candlemass (2nd February) but that is going a bit too far for me.

Orthodox Christmas is Jan 7. 

We are firm believers that our freedom of choice is as valid as yours. Take care of your own decorations please.

It Italy.......decorations stay up to the Epiphany......Jan. 6th. ----which is just as important  as part of the entire holiday- 

Manger scenes that show the wise men bowing down on the 25th,  are technically wrong...........the Epiphany us when the wise men supposedly showed up,  on Jan 6th.......according to the Legend,  and therefore they should NOT be placed in the manger scene , until they "arrive" , on the 6th.

That is also why kids are given toys and sweets on the 6th......to represent the 3 wise men arriving and bestowing their gifts. 

Of course , there's also the tale of the Befana witch, but that's another story. 

and then yes,  on the 7th.........it's time to take everything down. 

Some enjoy them longer. For myself, weather dictates when they come down. We have had years when they were buried in snow or frozen and couldn't be taken down. I'm not sure why it matters anyways, they can still be enjoyed year round if that is one wished.