Question

In general, court in the United States have repeatedly struck down efforts to include creationism or intelligent design in public school classroom. Why?

Answer 1

Because they are not supported by the results of any legitimate investigation
that's conducted in accordance with the Scientific Method. 

You may say: 
"Well then, teach both lines of reasoning,
and let each student decide for himself." 

This is suggested by the same people who aren't ready to let their
fourth-grader choose his own clothing, dinner menu, or school.
And it sounds reasonable to a vast mass of citizens who have decided
for them selves that the jury is still out on climate change.

What I'm saying is this:

-- The Scientific Method is a METHOD of investigation that's designed
and developed to remove the effects of human prejudice from the
collection and evaluation of evidence, and to be able to tell bogus
conclusions apart from true ones.  It's the most reliable way we have
of asking and answering questions about the natural world.

-- Some questions CAN'T be studied with the Scientific Method,
because experiments generally can't be constructed.  These include
matters of religion and faith.  Nobody can flatly state that those are
right or wrong.  We have no reliable way to say, either way. 
The only way to decide is . . . faith.   

-- It is illegitimate to take the answer to a question of faith that can't be
derived scientifically, and a scientifically derived conclusion, set them
down next to each other on the same table, and pretend that they can be
compared.

-- When you put them next to each other, say that they're equivalent,
and tell people "go ahead and choose one or the other", the situation
is bogus, the comparison is dishonest, and people who are untrained
or uneducated or immature are not qualified to "choose".

That's why.

This is my opinion.  I could be wrong.

Personally, I happen to be a believer.  But I cannot prove anything I believe
to anyone else ... not with rational argument, and not with evidence.  Those are
elements of the scientific method.  They're not applicable, and they don't work,
in matters of faith.