Because, because, because, because...
Why do you think we have three branches of government in the first place?
If members of Congress had been anticipated to be perfect paragons, the Founders would not have found the division of power to be necessary.
Congress will of course be susceptible to political pressure and public outrage.
The Court is there to rule on things that the other two branches do which may not be Constitutional.
I really don't know why you expect that Congress would not react to the angry howls of the people who elected it. Its function is to express the will of the people.
The people aren't always correct, especially when they're angry and frightened.