Paul MacKay writes:

>why is it forbidden to clap after each movement...

It is not "forbidden", but there is an unwritten law that it just isn't
done.  As Kevin Sutton wrote,'there is a mood established' by the music and
it just disrupts everything if there is inappropriate applause.  In opera,
I can understand applause (although it bothers me when it happens) after an
aria, not only giving the artist recognition, but also giving them a short
"breather".  But in a symphonic work or a concerto, when the music weaves
its magic spell, inappropriate applause really jolts me out of my reverie
and I resent it.  Wait until the end of the composition.

Tom Warren