Simple answer because God said we should not do it:
Amo 5:21 I hate, I despise your feast days, and I will not smell in your solemn assemblies.
You just took this verse out of context just as bad as when the Jehovah's Witnesses and other cults take things out of context. God despised their feast days because when they were celebrating them, God was not their focus. That is why. Here is the whole context.
Amos 5
Hear this word, Israel, this lament I take up concerning you:
“Fallen is Virgin Israel,
never to rise again,
deserted in her own land,
with no one to lift her up.”
This is what the Sovereign Lord says to Israel:
“Your city that marches out a thousand strong
will have only a hundred left;
your town that marches out a hundred strong
will have only ten left.”
This is what the Lord says to Israel:
“Seek me and live;
do not seek Bethel,
do not go to Gilgal,
do not journey to Beersheba.
For Gilgal will surely go into exile,
and Bethel will be reduced to nothing.[a]”
Seek the Lord and live,
or he will sweep through the tribes of Joseph like a fire;
it will devour them,
and Bethel will have no one to quench it.
There are those who turn justice into bitterness
and cast righteousness to the ground.
He who made the Pleiades and Orion,
who turns midnight into dawn
and darkens day into night,
who calls for the waters of the sea
and pours them out over the face of the land—
the Lord is his name.
With a blinding flash he destroys the stronghold
and brings the fortified city to ruin.
There are those who hate the one who upholds justice in court
and detest the one who tells the truth.
You levy a straw tax on the poor
and impose a tax on their grain.
Therefore, though you have built stone mansions,
you will not live in them;
though you have planted lush vineyards,
you will not drink their wine.
For I know how many are your offenses
and how great your sins.
There are those who oppress the innocent and take bribes
and deprive the poor of justice in the courts.
Therefore the prudent keep quiet in such times,
for the times are evil.
Seek good, not evil,
that you may live.
Then the Lord God Almighty will be with you,
just as you say he is.
Hate evil, love good;
maintain justice in the courts.
Perhaps the Lord God Almighty will have mercy
on the remnant of Joseph.
Therefore this is what the Lord, the Lord God Almighty, says:
“There will be wailing in all the streets
and cries of anguish in every public square.
The farmers will be summoned to weep
and the mourners to wail.
There will be wailing in all the vineyards,
for I will pass through your midst,”
says the Lord.
Woe to you who long
for the day of the Lord!
Why do you long for the day of the Lord?
That day will be darkness, not light.
It will be as though a man fled from a lion
only to meet a bear,
as though he entered his house
and rested his hand on the wall
only to have a snake bite him.
Will not the day of the Lord be darkness, not light—
pitch-dark, without a ray of brightness?
“I hate, I despise your religious festivals;
your assemblies are a stench to me.
Even though you bring me burnt offerings and grain offerings,
I will not accept them.
Though you bring choice fellowship offerings,
I will have no regard for them.
Away with the noise of your songs!
I will not listen to the music of your harps.
But let justice roll on like a river,
righteousness like a never-failing stream!
“Did you bring me sacrifices and offerings
forty years in the wilderness, people of Israel?
You have lifted up the shrine of your king,
the pedestal of your idols,
the star of your god—
which you made for yourselves.
Therefore I will send you into exile beyond Damascus,”
says the Lord, whose name is God Almighty.
Another reference that is similar to this has the same theme, a lot of it is through Malachi, where God calls out Israel on their disobedience against Him because they have hardened their hearts against Him and are only halfheartedly worshipping him, not in spirit and in truth but in a religious way and going through the motions. Including not following God's instructions. Sorry but you cannot use this verse to say that celebrating Christmas is wrong because according to that same logic then the Jews should not celebrate the feast days they celebrate like the Passover, Feast of Booths, Feast of Unleavened Bread, etc. God bless.
