Halloween and Christmas

Why doesn't muslims celebrate Christmas?

They don't celebrate Christmas because they don't believe that Isa (Jesus) is not the son of god. They believe that Isa (Jesus) is their prophet. Also it shows that he has a father but Isa (Jesus) in Islam doen't have a father. And it represents the cross but Isa (Jesus) in Islam is muslim.And in Islam Mary isn't called Mary she is called Maryam. But Isa (Jesus) is born in Bethlehem. And they are going against their belif.

Why doesn't muslims celebrate halloween?

They do not celebrate halloween because it is clearly haram. It is based on pagen traditions/events and it is linked to  Šayṭān (devils that are similar looking but more scary). And it is taking away your praying time when you say your intention.

 Holy Quran 2:168 explicitly warns people not to follow the Šayṭān, implying that humans are free to choose between the path of God or the one of Šayṭān.